Good games for co-op online (2 players).

Portal 2

Dead Island might be worth a shot.

Saint's Row 3/4

Torchlight 2 / Borderlands 2

Fear 3

Guns of Icarus Online. You'll have to pair up with two randoms or AI, but it's still really fun.

The games are both competitive and cooperative in nature, though, so if you want a game that's only co-op PVE, you'll have to look somewhere else.

PAYDAY 2, Saints Row 3 or 4, Left 4 Dead 2, Killing Floor, Magicka.

Saints Row The Third, developers added only 2 players maximum in coop, that is ultra stupid, but the game is cool.

Portal 2 would have to be the greatest coop challenge in my book.... it has 5 entire courses each taking around an hour

Trine 2

Far Cry 3

How To Survive. ( Oh and you can LAN or even local co-op i heard lol play Dead Island 1,2, etc but im not sure what u need,like controllers from Xbox 360 idk )

Dead Space 3

Risk of Rain, Resident Evil 5 (or any others but 5 is my favorite), Synergy, Mercenary Kings, Terraria, Dungeon Defenders, Orcs Must Die! 2

try spec ops mw2

Kane & Lynch 2, Transformers, Operation Flashpoint: Red River

dota 2

Agree with most of the above.

I know Minecraft isn't on Steam, but it's really fun with other people. Also Terraria is a blast. Last 3 times I had game night with my friend we played Terraria only.

Orcs Must Die 2 is also good.

Warframe

Unreal Tournament. My person favourite is 2004. Unreal 3 is pretty good but it lags a fair bit...

Fable 3

Portal 2

Awesomenauts it's kinda fun to play too xD

Lord of the Rings: War in the North is a very fun coop game playable with 1, 2, or 3 people and you can probably pick it up from anyone who has an extra copy from the Humble WB Bundle if you didn't get it

Fear 3.