Demon-Killing Knife

The Demon-Killing Knife/Ruby's Knife was first seen in the possession of Ruby in "The Magnificent Seven." It was used to kill the Seven Deadly Sins. The Winchester brothers 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.

Powers
This knife is capable of killing lesser demons, however it cannot kill the more powerful demons, but Crowley, the King of Hell, was fidgety in its presence. Since its introduction, it has become one of the Winchesters' most common means of killing demons.

Immunity
Visually, in "I Know What You Did Last Summer", Alastair, after getting stabbed with Ruby's Knife, he remains unaffected. However, in 4X16, he implies that if stabbed perfectly in the heart, he will die.

Alastair is the only demon to date to be shown immune to the blade, although it is unknown if Azazel or Lilith would be immune too, as they're more powerful than him.

Origin
The exact origins of the knife and how it came to be in Ruby's possession are unknown.

It may have been created by Samuel Colt, who had it in his possession during his lifetime along with the Colt. A deleted scene from "Frontierland" shows Samuel showing it to Sam when the latter came to ask for the Colt.