Best co-op games on steam?

Most over-looked yet amazing coop game is Transformers: War for Cybertron. As a general rule I stay away from most licensed games because they are crap, but this one really impressed me. 3 player coop, 2 campaigns (Autobots and Decepticons).

The only co-op game that I can think of that hasn't been mentioned yet is Resident Evil 5. I got a bit put off Resident Evil after playing the last couple of games on consoles, but tried this one for the co-op on PC and have been having fun with it.

Other than that, I'd like to add my vote for Saints Row The Third and Dawn of War II.

ahh, try Trine 2 :)

killing floor

PAYDAY THE HEIST. some say that left 4 dead is better, but this game is 100 percent worth it. i like it more, but thats just me....

Portal 2, Titan Quest

orcs must die 2!

BORDERLANDS 2