The Best Co-Op Games on Steam? (Local/Online)

Clicky

Don't own them on Steam, but the Lego games are great fun in Local Multiplayer.

As for online, there's quite a few obviously with Coop features implemented very often nowadays, so a special one that imo is designed to be played in coop is Lara Croft: Guardian of Light (also don't own it on Steam but played online coop on 360)

Of course you can also engage in jolly cooperation in Dark Souls 1 & 2 ;)

Had a blast with Monaco and still having it with Battleblock Theater! Strongly recommended

Left 4 Dead and Borderlands (2) are great games too

Castle Crashers is fun (had it on Xbox a few years ago) and I've heard good things about Battleblock Theater (same creators).
Portal 2 has great singleplayer and multiplayer.
Borderlands (2) are fun single player but IMO better multiplayer.
Dungeon Defenders is good but multiplayer is pretty dead.
Worms Armageddon is fun, local or online. EDIT: sorry, Worms isn't really co op but still fun mutliplayer.

Killing Floor.

Best coop Borderlands 2

I wish there would be more story based coop games, cause I prefer playing coop over multiplayer.

payday 2 is a great game too ;)

Tons of awesome co-op games on Steam, here are some that I've enjoyed:
Trine 2
Sanctum 1 & 2
Orcs Must Die! 2
Dungeon Defenders
Portal 2
Terraria
Risk of Rain
Awesomenauts
Torchlight II

Terraria, Borderlands 2, Dead Island (actually enjoying it a lot), Just Cause 2, Magicka, Saint's Row III & IV, Portal 2, Minecraft and Starbound.

Portal 2, L4D2, Borderlands 2, Payday 2

Borderlands 2 best co-op game ever. But Terraria, Dead Island, Castle Crashers... Have good co-op.

Left 4 Dead 2, Pay Day 2, Dota 2, Team Fortress 2, Portal 2, and any other game ending on 2

Borderlands 2, L4D2, Killing floor and Warframe

All mentioned above and I would add "Hammerwatch" too, had fun playing this one. Also "HAWKEN" and belive it or not "Orion:Dino Horde"

Saints Row games have good co op