Sammael

Demonic Possession - Like angels (besides Cupid, or so it seems) , most demons (only known exceptions are Acheri, Daeva and Hellhounds) require a human vessel or "meatsuit" to travel on Earth. Unlike angels, they do not need a person's consent. Also unlike angels, demons can possess corpses. It is easier to take control of people who are in states of heightened emotions or fear. When possessing a human, the Demon invades their muscles, bones and brains and can also access their host's memories while possessing them (even if it is a brief possession). This ability can be thwarted by Anti-possession tattoos. Also under certain circumstances, the person being possessed can regain control if his or her will is strong enough. They can also possess humans who are already possessed by an angel (and probably by another demon), but the only problem is the angel can neutralize it, because angels are naturally stronger than demons. In many cases, the demon will ride the host hard for fun and once they vacate, the host will die.
Electromagnetic Interference - Demons, like spirits, often disrupt nearby electronics with their presence. Large numbers of demons can cause thunder storms.
Flight - Only when in their smoke forms.
Immortality - Demons have an indefinitely long lifespan and an arrested aging process.
Invulnerability - Injuries that would be fatal to humans, such as broken necks or gunshot wounds, have very little - if any - effect on demons. They can only be killed by special weapons or rituals. Unlike angels they don't always heal their vessels. If the "meatsuit" receives too much damage they simply find another human to possess, leaving their former vessel to die.
Super stamina - Demons never tire, and do not require food, water, oxygen, or sleep to sustain themselves. Sam states that they also do not get hot or cold and Dean states they do not need to urinate.
Super strength - Demons possess superior physical strength compared to that of humans; capable of physically overpowering humans. Demons can also overpower most monsters. Once, a demon (the Phantom Traveler demon) ripped open an airplane hatch with over 2 tons of pressure at over 1,000 feet above the ground. The higher ranking the demon, the stronger it is. The highest ranking demons can overpower low-level angels.
Supernatural Perception - Demons are able to see hidden supernatural activity and beings, such as Reapers, ghosts, and hellhounds.
Supernatural Senses - Demons have an enhanced sense of smell and taste, demonstrated as Crowley could identify Castiel was in Dean's Impala and that he and Raphael had been using dog's blood. A demon possessing a waitress claimed she could smell Sam's soul.
Telekinesis - Demons can move or suspend objects and other beings with their minds. Higher ranking demons can use this ability to pin their victims to walls or kill people by snapping their necks.
Soul Extraction - Demons have the ability to forcefully remove a person's soul from their body.
Biokinesis - Some demons can manipulate the biology of other beings. Azazel, for example, was able to torture Dean by causing him to hemorrhage. Meg was also able to cause Crowley to cough up blood in a similar manner. Astaroth stopped Elizabeth's heart with a wave of her hand and Abaddon caused people to bleed from their eyes. Crowley was even able to cause a girl to explode by snapping his fingers.
Electrokinesis - Displayed by Azazel and Crowley. They could manipulate electronic equipment to their will. Also the demon possessing Isaac's corpse was able to switch on an old radio.
Invisibility - Some demons can become invisible to humans.
JessPyro
Brady kills Jessica with pyrokinesis.
Pyrokinesis - Some demons can generate and manipulate fire. Azazel, Abaddon and (probably) Brady have demonstrated this ability. Crowley can also manipulate fire; he has lit a fire place by raising his hand, extinguished a Holy Fire circle by snapping his fingers, and also pretended to set himself on fire.
Telepathy - Dean mentions that some demons can read minds. Envy used this ability to gain intimate knowledge about Tamara's past without ever encountering her before or possessing her.
Soul Reading - Some demons, particularly Crossroad Demons, can assess the condition of one's soul.
Spell Casting - Some demons can practice witchcraft, much like witches, like altering the range/space they occupy. Other demons are adept in Enochian magic which they use to protect themselves from Angels. Alastair knew of a spell that could allow him to banish an angel back to Heaven. Meg and Casey knew of spells that they could use while inside a Devil's Trap, allowing them to break free. Astaroth was extremely skilled in magic and used it to kill witches, almost kill Dean and severely hurt Ruby.
Teleportation - Some demons can travel from one place to another instantly, without occupying the space in between.
Terrakinesis - Some demons can create small earthquakes. This was once used to destroy a Devil's Trap drawn by Kevin Tran. Abaddon was also able to create an earthquake with her presence that broke a Devil's Trap restraining Crowley.
Voice Mimicry - Crowley was able to mimic his original hosts voice while possessing Mrs. Tran in order to taunt the Winchesters. Though never confirmed, it is hinted at that Ruby may have been the one that sent Sam a voice text of Dean, distorting what Dean would have really said.
Thermokinesis - Certain demons such as Lilith and Crowley can emit great heat through their skin resulting in severe burns to whatever they touch. Lilith used this power to scald Sam when forcefully kissing him and later to burn through a Devil's Trap. Crowley once caused Ruby's knife to burn red hot by merely snapping his fingers, forcing Dean to drop it. (We Need To Talk About Kevin).
Healing - Used by Crowley, he is the only demon who has shown the ability to heal others out of the context of a deal, albeit a small wound. (All Dogs Go To Heaven)
Weather Manipulation - Some demons such as Meg and Father Gil are able to alter the weather to a limited extent such as creating sudden gusts of wind. More powerful demons like Azazel can create stronger winds and Abaddon could trigger thunder and lightning merely by screaming.
Regeneration - At least some demons can heal their hosts from any physical injury they sustain. Meg was able to heal several broken bones after falling from a window. Crowley, Meg and Ruby have all been shot on several occasions and show no signs of injury following the incident. Crowley has also healed his vessel after being severely burned by an angels white light and being stabbed through the shoulder by an angel blade. However, after leaving their host, any injuries the body has sustain reappear and the host dies. Oddly, while Crowley has vacated his vessel several times, many of his injuries (such as his burns) did not reappear on his host (though his host seems to have died so it is possible some of his injuries did reappear).
Mind Control - The Seven Deadly Sins each possessed the ability to force humans to carry out lethal actions based around the Sin they represented. For example, Envy forced a woman to kill over a pair of shoes, Gluttony forced Issac to drink drain cleaner and Sloth caused a family to stay seated and starve to death.
Access to/from Hell - High ranking demons such as Azazel, Alastair, and Crowley can easily teleport themselves back and forth between Earth and Hell, without the need for any complex summoning ritual or assistance. This however, varied among high-tier demons, as some were kept imprisoned due to unknown reasons and methods.
Dream Walking - Displayed by Azazel only on the special children. He could enter and control the dreams of his targets.
Mind Manipulation - Displayed by Crowley only, he was able to grant his illiterate son the ability to read simply by touching his head.
Enhanced Immunity - As a general rule, the more powerful the demon, the more resistant they are to harm and common anti-demon methods such as salt and holy water. High ranking demons like Alastair are immune to the Killing Touch of normal angels and both he and Abaddon are immune to the killing effects of the Demon-killing knife though it causes them pain. The Knights of Hell could only be killed by the First Blade and Cain, the original Knight, is completely immune to the knife to the point it doesn't even hurt him.
Memory Manipulation - Displayed by Crowley and Azazel. Crowley stated that he "scrubbed" Kevin's short term memory and Azazel was able to erase Dean's memory of his conversation with Tessa. (In My Time of Dying)
Summoning - Displayed by Samhain. He could summon and control various monsters, ghosts and zombies.
Exorcism - Displayed by Abaddon and (possibly) Lilith. She forced a demon out of its host and back to Hell by choking its vessel. Lilith also claimed that she sent Ruby to Hell and then possessed her vessel.
White Light - Used by Lilith (Jus In Bello, No Rest for the Wicked) and Samhain (It's the Great Pumpkin, Sam Winchester). High ranking demons can use this to generate massive amounts of destructive energy that can obliterate large areas. This ability is extremely rare and are otherwise present only in the more powerful angelic beings.
Memory Extraction - By inserting part of her essence into her victim, Abaddon could access their memories (even their subconscious memories) and gain information about what they had seen and heard.
Demonic Smiting - Cain, one of the first and most powerful demons, displayed the power to smite other demons similar to angels, but with a red glow rather than a white one.
Special Abilities when making deals with mortals
Healing - In the context of a deal, crossroads demons can heal themselves and others of any wound as demonstrated by Azazel and Crowley. While in Hell, demons such as Alastair can heal any wound applied to their victim, so that they can continue to torture them again and again.
Reality Warping - Crossroads demons, and a few other Demons can grant humans a wish in exchange for their souls. It is worth noting that Azazel did not need to take souls, and could grant wishes from his own power, which he did in exchange for permission to enter the homes of special children, on their 6-month birthday. Lilith also claimed she could make deals without taking a soul but this was never confirmed. While in Hell, demons can alter reality to torture people in ways that are beyond human imagination. One of the reasons Alastair despises being on Earth is because reality is "too concrete" and there are too few resources for torturing people effeciently.
Resurrection - One of the wishes demons can grant is the resurrection of a loved one. Crowley claims that, since he is now the king of Hell, he is strong enough to resurrect anybody in his care without a deal. He demonstrated this by resurrecting Samuel Campbell in Season 6; the 'patriarch' he 'took off the bench' in order to capture Alpha creatures, which Crowley hoped would lead him to the door of Purgatory. Azazel also displayed resurrecting, the young John Winchester after he killed him, in the context of a deal. He couldn't however resurrect Dean (possibly because he hadn't crossed over yet)- instead, he possessed a Reaper, who he used to do it for him.
Shapeshifting - Demons can take any form they wish while in Hell. Several demons were able to take the appearance of Sam and Dean while torturing Bobby Singer in Hell.

Devil's Trap - A Devil's Trap will hold a demon in place and prevent its escape. No matter how powerful the demon is, they are bound by the Trap. The Trap will also demons from using their powers. Castiel's Enochian circle was able to completely bind even Alastair's abilities. It also locks a demon inside its host unless it is exorcised. For some reason, a demon inside a trap is usually unable to do anything to damage the integrity of the circle, which would allow its escape. A Devil's Trap engraved into a bullet and shot into Abaddon's head was shown to limit her movements like a normal Devil's Trap would and to bind her to the body she was possessing.
Salt - Demons are unable to cross a line of salt and can do very little to damage the line at all. They cannot open doors lined with salt if opening it would break the line. Salt can also be used to torture demons by making them ingest it or by injecting them with saltwater. Forcing a Demon to ingest salt will drive them out of their host. It can also be put into shotgun rounds to hurt and knock back demons. However, Azazel is immune to this weakness and Crowley was hardly affected by rock salt shots, but he still found the experience painful.
Holy Water - Holy Water burns demons that come into contact with it, like acid (though it does no damage to the human host, only the Demon). It has proved to be very effective against demons, as only Azazel and Lilith have been thought to be immune to it. However, certain high-tier demons seem to develop some resilience to it as Ruby practically laughed when Sam was about use it against Alastair because it would barely slow him down and, while it caused them pain, it only affected Crowley and the Seven Deadly Sins for a very brief period of time.
Iron - A demon can't touch iron without its skin burning or cross it if it is used to erect a barrier, much like salt. Samhain and Alastair were immune to this weakness and Astaroth had some resistance to it. In large groups, Demons are able to overcome this weakness, as the Demon army was able to break an iron Devil's Trap in order to escape.
Palo Santo - Basically a wooden, hybridized version of salt and holy water. When a demon is stabbed with it, it causes the wound to smoke and painfully pins the demon to a single spot, immobilizing them long enough to perform an exorcism. Palo Santo isn't commonly used by many hunters due to the fact that it damages the hosts the demons are possessing. (The Magnificent Seven)
Flesh Sigil - By carving a particular sigil into a demon's possessed flesh, it is possible to lock the demon within its host and disable its powers, the most important one being teleportation. However, demons with these sigils are not bound to a single place and can roam around freely if not restrained by some other means. (The Devil You Know)
Holy Fire - Though not fatal to demons as it is to Angels, Holy Fire can cause demons extreme pain, as demonstrated when Castiel threw Meg into Holy Fire (Abandon All Hope...) and when Sam doused Abaddon in Holy Fire, causing her so much pain she fled both the scene and her vessel (Sacrifice). Another demon committed suicide by dousing himself in Holy Fire (Soul Survivor)
Purified blood - The blood of a person who has confessed his/her sins or is blessed by a priest is shown to be able to hurt demons if injected into them. This hurts Knights of Hell more than regular demons.
Angels - Demons are inherently weaker than angels and therefore, are highly susceptible to the powers of angels. Even low-tier angels are able to easily overpower and kill almost all demons except the most powerful ones, demonstrating the apparent disparity in power between the two supernatural species. Only the highest ranking demons in the hierarchy of Hell could fight and overpower low-tier angels. Furthermore, in extremely large groups, Demons can over power regular angels, as shown when Crowleys army defeats Castiel's guards. However, even the most powerful demons are completely no match for Archangels, who can easily destroy them.
Death - Being the concept of death literally personified, Death can effortlessly destroy any amount of demons instantly.
Exorcism - Will pull the demon out of its host and send it to Hell. Hosts may not survive. Before Sam and Dean learned that demons could be killed by the Colt or Ruby's Knife, exorcism was the only known method used by hunters, to stop demons. Reciting the exorcism backwards will force the demon back into its host. Very powerful demons are immune to exorcism (eg. Abaddon).
Eve - As Eve is the primordial monster and mother of all monsters, she is able to kill even the most powerful demon. She has demonstrated the ability to even block the powers of angels, further displaying her power.
God - Being able to do anything, God can effortlessly destroy any amount of demons instantly.
Hellhounds - Although normally docile towards demons, hellhounds can overpower and kill demons just as easily as they can kill humans.
Leviathans - Leviathans can easily slaughter any type of demon. Dick said if he wasn't busy with other things, he might've wiped out all of demonkind.
Physical Damage - Though demons themselves cannot be harmed by conventional means, severe damage sustained by their vessels (such as severed limbs) will greatly reduce their effectiveness in combat. Unlike angels, most demons are unable to heal their vessels. As such, if the vessel accumulates too much damage, the demon will be forced to vacate its host and search for a new vessel. However, some Demons choose to heal their host's injuries but this is relatively rare.
Ruby's Knife - The knife originally belonged to Ruby. It will kill a demon that is within a host if the wound dealt is a fatal one. Upper level demons are more resistant to the pain of the knife, and the knights of Hell cannot be killed by the knife. The only demon that the knife had no effect on was Cain. the only definitive examples being Alastair, Abaddon and Cain. However most demons (with the exception of Cain) find contact with the knife extremely painful even when not lethal.
The Colt - Much like Ruby's Knife, it will kill any demon that is shot while within a host if the shot is a fatal one. It is more powerful, however, as it affects upper level demons as well, even when the Knife does not.
Bones - If the bones belonging to the demon are burned, the demon will be killed much like a vengeful spirit. This can also be used to torture demons. This until recently was believed to be merely a rumor, however Bobby tested this method against a Crossroads Demon resulting in her death. Crowley may have been aware of this method as he tried to convince Bobby it was "a myth". This tactic is very difficult to use; as it requires to know the location of a demon's bones, it may be necessary to have extensive information on their human biography, and many demons themselves can no longer remember they were ever human at all, let alone particular details of their earthly life, including their names.
Death's Scythe - Death's Scythe can kill demons, as it is capable of reaping just about anything. Crowley confirms this when he gives Dean the scythe. (Two Minutes to Midnight)
Ingestion of Salt - Under some circumstances, the ingestion of large quantities of salt can force a demon to vacate its host. (I Believe The Children Are Our Future)
Unknown Virgin Sacrifice Spell - Ruby claimed to know a spell that would destroy any demon within a one mile radius. Not much is known about the ritual except that it requires cutting the heart out of a virgin. (Jus In Bello)
Upper Tier Demons - Crowley killed all but one demon from a demon hive in season five, so it can be assumed that a more powerful demon is able to destroy lower level demons. Many demons have also feared that their masters, such as Lilith or Azazel, will kill them if they betray them. However, this may have been through the use of a weapon as Crowley, Meg and Abaddon all later use an Angel Blade to kill demons. Cain however possessed the ability to smite lower demons in a similar manner to angels. In Hell, Crowley displayed the ability to turn demons to dust with a snap of his fingers, however, this seems to be limited to him in Hell as he still relies on Angel Blades on Earth as do other demons in Hell.
Demon Bomb - The Word of God tablet on demons describes a spell which can completely destroy demons. Kevin Tran is able to cast this spell and it is shown to work, disintegrating two demons and their vessels. (We Need To Talk About Kevin) Following instructions Kevin made in a notebook, Sam and Dean are later able to turn this into a bomb that Sam uses to kill three demons. (A Little Slice of Kevin) Sam and Dean later use it to kill four demons while rescuing Samandriel. (Torn and Frayed)
Demon Cure - It's possible to turn a demon into a human again if, in a sacred ground, a human injects his own "pure" blood in a Demon. The blood must be given hourly for eight hours. In the last dose, the blood must be fresh and the donor must tell before the exorcism: "Exorcizamus te, omnis immundus spiritus, hanc animam redintegra, lustra, lustra". (Clip Show) Alternatively, the curer only need inject the purified blood and it will cure the demon. (Soul Survivor)
Angel Blade - In Torn and Frayed, it is shown that an Angel Blade can be used to kill a demon. Sam uses one to kill two demons while rescuing Samandriel. It was earlier implied when Castiel drew his blade against Crowley. Crowley and Meg use Angel Blades to kill other demons when necessary. Even Abaddon has been shown to use an angel blade on occasion (Road Trip) and it appears to be Crowley's method of executing demons who have committed crimes against him. (Soul Survivor)
Archangel Blade - Even more powerful.
The First Blade - The First Blade, combined with power from the Mark of Cain, is capable of killing demons that even other weapons can't such as the Knights of Hell.