Some good co-op games on steam?

Dungeon Defenders, Left 4 Dead, and Terraria would be my picks.

Killing floor, payday

Dungeon Defenders
Orcs Must Die 2
Awesomenauts
Realm of The Mad God
Terraria
Audiosurf

Trine 2 obv.

Torchlight 2, Portal 2, Sanctum 2 :< Hmm, I see pattern there...

Shooter/RPG - Borderlands 1 and 2, Dead Island, Riptide , Lords of the Rind: War in North , L4D2 ins't bad at all.
RPG - Dungeon Siege 1, 2 , 3, Torchlight 1 and 2.
Defense - Orcs Must Die 2, Dungeon Defenders, Sanctun 1 and 2.
Action - Warframe, Killing Floor, God Mode.
Other - Dawn of War 2, Guns of Icarus if you have 3+ people, Portal 2, Worms Revolution, Terraria it's not bad at all.
Dead Rising 2 have a coop mode with the DLCs if I'm not wrong.

• Heard or Guess to be cool to play at Coop but din't had the chance to play yet. (Invites to are welcome =D)

Nazi zombie snipers elite. Have so much fun with friends.. Also arma . And days mod which is not a story coop BUT You guys can survive together by having each others backs or whether you want just be a bandit and kill on site :D

Killing Floor, Left 4 Dead (1&2), Portal 2, Payday (1&2), Saints Row (2&3), Castle Crashers

payday 2

System Shock 2

Orcs that must die

Resident Evil 6

Don't forget the Half-Life mod Sven-Coop. :)

payday 2 :D

I just came to tell you that your avatar is awesome! Love MMF! :))

magicka, orcs must die 2, trine 2, portal 2, torchlight 2, dungeon defenders, dungeonland. All of them are awesome for coop in my opinion.

Dawn of War 2 series

Men of War: Assault Squad

Synergy

Killing Floor of course