Best co-op games on steam?

Portal 2, Left 4 Dead 2, Borderlands 2 and Killing Floor.

Borderlands / Borderlands 2
Portal 2

Would you believe those are all I know?

And ****, ninja'd

I would recommend Saints Row: The Third, it's quite fun playing it with your friends.

Payday

Aside from the previously mentioned Portal 2, I really enjoyed both Trine 2 and Lara Croft and the Guardian of Light in coop.

Sanctum and Torchlight 2

Aside from the ones mentioned already- The Darkness 2 has up to 4 players co-op. (It's rather short but has some replayability.)
Also Shoot Many Robots (<=4 players) and F.E.A.R. 3 (2 players).

Saints Row 3 is pretty fun for co-op.

Borderlands 1/2 as well but those are already mentioned.

Other suggestions:

• Castle Crashers
• Magicka
• Dungeon Defenders
• DoW II Retribution
• Orcs Must Die 2

[Most] of these haven't been mentioned yet and I've had a lot of fun playing them with friends. Hopefully this offers quite a good variety. Of the ones below my favorites are Sol Survivor, OMD2, Serious Sam, Magic, and Zero Gear. Notably both Zero Gear and Serious Sam are going to be a lot more fun with more friends.

Shooter: Serious Sam (BFE, SE, FE), Shoot Many Robots
Tower Defense: Orcs Must Die 2 (2 player max though), Sanctum, Dungeon Defenders, and Sol Survivor
Strategy: Civilization V, Magic DOTP 2012/2013
ARPG: Torchlight II, Magicka, Darkspore
Other: Wanderlust Rebirth, Zero Gear

Magicka is actually fun with a group of friends, much rage to be had. :)

Something not mentioned yet, Terraria.

That I know: Killing Floor.

Splinter Cell Conviction.
Try the new Sniper Elite (Nazi Zombie) it is really fun.

Definitely Serious Sam the first and the second encounter.

why not try synergy? Its free and its entire half life 2 campaign (do have to own half life 2 though)

pay day,first!!then trine 2 and then lara croft and the guardian of light.

Games people haven't mentioned:

• Jamestowns up there somewhere.
• Fear 3's survival modes are fun as hell.
• Burnout Paradise, open world, cruise, do challenges in MP.
• Look up Warframe/Path of exile/Firefall for some free coop goodness though firefall is an mmo.

lost planet 2

Sanctum

Sniper Elite Nazi Zombie Army (=

Alien Swarm !

Sniper Elite: Nazi Zombie Army is intensely fun!

Not sure if this is what you had in mind but here goes: Dota 2, AriMech, Tribes: Ascend (these are all free to play btw). Also, Renegade Ops immediately comes to mind, Red Faction: Armageddon maybe.

Left 4 Dead, Payday, Magicka, Borderlands, Splinter Cell Conviction, Portal 2...

Left 4 Dead 2

Castle Crashers and Rayman orgins.