Demon-Killing Knife

This knife originally belonged to Ruby and is capable of destroying most demons. Sam and Dean gained control of the knife and are in current possession of it. Meg and Castiel have also been known to make frequent use of this blade. Since its introduction, the knife has become one of the Winchesters' most common means of killing demons.

Physical appearance
At first glance, the knife appears to be an ordinary hunting knife with a curved, serrated-edged blade and a wooden handle. However, upon closer inspection, symbols can be seen engraved along the blade.

Powers and Abilities
This knife is capable of killing lesser demons, however it apparently cannot kill the more superior demons, though Crowley, the King of Hell, was fidgety in its presence, as was Lilith. Castiel, being an angel and not a demon, was completely immune to the knife. As well as killing demons, non leathal cuts are excruciatingly painful for demons, much like ones from angel blades are to angels.

The demon Alastair survived being stabbed by the knife, but implies that if he was stabbed directly through the heart with it, he would die. However seconds later, Castiel telekinetically moved the knife down towards his heart, but all this did was cause Alastair more pain, suggesting he was truly immune. The knife is also capable of harming the Horsemen War, Famine, and Pestilence. To most other beings, it appears only as a sharp knife.

Immunity
Alastair was the one of the few demons to date to be shown immune to the blade. Samhain also showed resistance to the cuts from the knife, finding them only uncomfortable, not antagonizing, but its unknown if he too was immune to its kill power. It is unknown if Azazel or Lilith would be immune too, as they both were more powerful than both of them. However, Lilith is shown to be frightened of the knife when Sam threatens her with it, so it is possible the knife could kill or at least harm her. However, as Alastair is her subordinate and that the knife's origin is probably unknown to demons (as Alastair was unfamiliar with the design) it most likely cannot harm her but she wouldn't risk it as it would mean she would be unable to break the final seal.

Abbadon was also shown to be immune to the powers of the knife, therefore so were the other Knights-of-Hell--a mysterious group of very powerful demons created by Lucifer. This group of elite demons could only be killed by Archangels. However, like Alastair, the knife was capable of hurting Abaddon, she (and presumably the other Knights-of-Hell) were still weaker to Knife than Alastair, as it knocked her down, while Alastair only found it painful.

Trivia

 * A deleted scene from Frontierland shows Samuel Colt in possession of the knife. Therefore, he may have invented it, just as he invented the Colt.
 * In the novel War of the Sons, Abbadon calls it a "cursed knife" and merely laughs when the Winchesters threaten him with it.
 * In episode 12 of season 8, Henry Winchester notes that to kill a demon one would need an "ancient demon killing knife of the Kurds," showing the origins of the knife as Dean says that is what the knife is and Henry reacts with surprise at the sight of it.