What are some amazing co-op games that you've played? (on Steam)

L4D2

Hmm, for co-op games, I don't know many myself, having only played Left 4 Dead and Borderlands with friends.

And if you don't mind a non Steam title, I highly recommend Obscure. ****ing awesome game.

Payday

Serious Sam Classics: The first and the second encounter.

Saints Row: The Third. Compete with your friend to see who can create the sexiest character.
EDIT: Sanctum has co-op too, as well as the ability to rocket jump. :)

Borderlands 2, Torchlight 2, Dungeon defenders, Dead island

killing floor

Orcs Must Die 2!!!! I got hooked, 15 hours in 4 days.

Castle Crashers.

/thread

Payday: The Heist, Dungeon Defenders, Borderlands, Castle Crashers and Warframe.

Splinter Cell Chaos Theory. You won't find many pure horror games with coop.

Portal 2

Trine 2

Aliens: Colonial Marines.

Borderlands 2

Portal 2

Saint's Row 3, best co-op game ever.

Borderlands 2 is a must.

portal 2,maybe adventures of shuggy? (has co-op)

Portal 2 the best for me (f*ck Borderlands and their gamespy thing).

Alien Swarm? :<