Rings of the Horsemen

These items are astronomically powerful, and each of the Four Horsemen of the Apocalypse owns one that allows them to channel their essential abilities, although they can still execute some powers without it on. They prove integral in the apocalypse. Brady reveals that War and Famine have been "shut down" so to speak without their rings. These rings conjoined make the key to Lucifer's cage. It was recently revealed that if a human were to wear one, they would get that Horseman's power until removing it.

War's Ring
War's has a golden ring. War used the ring in Rufus's town to cause the inhabitants to hallucinate a demon attack. The brothers were able to stop him by cutting off his ring finger.

Famine's Ring
Famine's ring is silver and has a round black stone on it. He uses the ring to spread his influence all over a town, making people kill themselves (and each other) over whatever they craved so he could feed on their souls. Famine was eventually weakened by Sam's demonic powers and his ring was taken.

Pestilence's Ring
Pestilence's ring is also silver, like Famine and Death. It has a round green stone on it. He uses this ring to spread diseases in a hospital to test their effects on humans. Castiel cuts off two fingers, separating him from the ring.

Death's Ring
Death's ring is silver with a white stone on it. He uses this ring to channel his immense power (it is yet unclear what the full extent of his power is but he can kill anyone/anything instantly as well as raising the dead) although he does not appear to need it. In "Two Minutes to Midnight", he gives the ring to Dean without any apparent side effect to him.

The Keys to Lucifer's prison
Gabriel revealed that the Horsemen's rings held a secret power that not even Lucifer knew about. They are, in fact the keys to Lucifer's original prison and when combined they have the powers to reopen his cage and imprison him again.

Lucifer, unlike what Gabriel thought, is aware of their ability to open his cage.

Trivia

 * War's ring is the only one without a stone and it is the only one made of gold.
 * Death's ring has a square stone instead of a round stone, like Famine and Pestilence.
 * All the horsemen's rings except for War's are made of silver.
 * Death apparently does not suffer any side effects from giving his ring to Dean, although it could be because it was done voluntarily and not taken by force, like his brothers'.
 * Due to the fact that Dean gained Death's abilities upon putting on his ring, it seems quite likely that this would also apply for the other three rings. Although no human, other than Dean, has worn a Horseman's ring.
 * Death, Famine and Pestilence's stones in their rings are the same colour as their horse (or car)