good coop games?

Borderlands 2

Payday The Heist

Payday 2

Chivalry

Awesomenauts

Dota 2

Alien Swarm

Torchlight 2(Has an 8 player mod I think...?)

Borderlands GOTY

Killing Floor

L4D2

Dead Island

Orcs must die 2 ?

Magicka and
Castle Crashers too i think

Serious Sam HD: The First Encounter and Serious Sam HD: The Second Encounter. Minced meat FTW!

take a look at this looks good but havent tried it yet ;)
http://store.steampowered.com/app/237590/

Ghost Recon: Future Soldier (up to 4 players)

Surprisingly I've had the most fun with mods. Try the Far Cry assault co op mod, sven co op for Half Life and Synergy for Half Life 2.

EDIT: Minecraft, Terraria, Left 4 Dead, Diablo 2 and Torchlight 2, Killing Floor and I haven't tried it but I heard the Arma games are really fun for co op.

Warframe.

dungeon defenders

Borderlands 2,
Killing floor,
left 4 dead 2,
payday,
terraria,
serious sam series,
sniper elite nazi zombie,
torchlight 2,
castle crasher,
dungeon defenders,
just cause 2 (multiplayer mod)*,
god mode

sniper elite: nazi zombie army (4 players)

GUNS OF ICARUS ONLINE

Portal 2 has the best coop, at least for me

Monaco what's yours is mine

Magicka

portal 2 co-op is definitely worth trying

4 players........ dunno, try borderlands maybe?

Survivers (Slender Coop)
World of Warcraft can be a coop game
Call of Duty Modern Warfare 3
Saint's Row 3

Saints Row 3 is probably the most fun I've ever had playing co-op in a game.

a minor game could be bunch of heroes

Castle Crashers <3 it's only 4 players but too fun!

Little Fighter 2!

Local multiplayer is best multiplayer, so - Rayman Origins. Oh, and Jamestown.
For online, go with Borderlands.