Some good co-op games on steam?

On-line or off-line co-op?

Castle crashers - lots of fun

portal 2 . Cursing and laughing simultaneously.

left 4 dead 2

Saints Row, Trine 2, Sanctum 2, Payday, Monaco, Torchlight 2, and Orc Must Die 2.

Sanctum, Payday, anything Lego.

The Cave

A kind of obscure game that is pretty fun, that I've enjoyed so far, is Greed : Black Border.

portal 2, i only really play co-op and this is by far the best game i have played, my partner in science and i had so much fun, after you go through the story there's all the workshop mods to play, 1000`s of maps, amazing game.

Dark Souls, you will need to co-op very well with your left and your right hand.
But really, try Sanctum (or Sanctum 2)

Resident Evil 5, Company of Heroes (not strictly co-op, but can be played co-op and is awesome), Magicka.
+1 for Trine 2, Portal 2 and Orcs Must Die 2.

These are all great games that you should be playing with your friend(s).

Borderlands

Hammerwatch

Payday 1 & 2 are great, especially if you like tough and tactical
There's a co-op aspect to Team Fortress 2 which is F2P
Boarderlands 1 & 2

Depends on who do you want to play with. :P

Lord of the Rings: War in the North
Resident Evil 6 (It's better than 5, don't go with the mindless hate if you can tolerate 5, 6 is better. Both are a terrible Resident Evil game but both are pretty fun for co-op)
Shoot Many Robots
Dead Island and Riptide
Saints Row 3&4
Sacred 2 Gold
Trine 1&2
Rainbow Six Vegas are co-op I think
Terraria
Renegade Ops
Magicka
Hunted The Demon's Forge

and then of course what everyone else has already said.

EDIT: Sorry about lack of commas, forgot SG hates lists...

Killing Floor - Which I just found is included in the Indie Gala!

Dark Souls

Hunted: The Demon's Forge

Borderlands

Left 4 Dead

Saints Row: The Third.

Sanctum.

Borderlands.

Terraria.

Guns of Icarus Online (A steampunk airship combat game has a crew of 4 (pilot/engineers/gunner) per ship and 2-4 ships on each side. It's got a pretty good community.
Or if you just want FPS Borderlands is great (I played it splitscreen on 360 but sure on steam is still fun).
Also Have a look at co-optimus listshere. Most of these should be on steam and you can filter by online/splitscreen/lan co-op etc and sort by rating (they have a separate rating for co-op).

Party of Sin. At first comes across as a Trine rip-off, but is actually more entertaining in my opinion. It's hard to tell with just an image, but the wiggle animation for this first boss was hilarious PoS narwhal

And as others mentioned one of the best co-op games I've ever played.. terraria.

Worms Reloaded

Borderlands 1-2; Saint's Row series

Alien Swarm & Zombie Panic Source are pretty fun F2Ps.

Shank! For me, Co-op was the star of this game.