Co-op Games

Try Guns of Icarus Online
Not exactly a shooter (though it involves well shooting down other ships) coz you can also be a pilot (drive the air-ship) or engineer etc. etc.

http://www.co-optimus.com/system/4/pc.html

Monaco.

Ghost Recon Future Soldier

Portal 2

modern wushfare 3

virtua tenis, pro evolution soccer, fifa,torchilight2, diablo 2, chess, racing games,etc..

Terraria.

terraria, castle crashers, magicka, torchlight 2, civ?

Here are a few titles:

• Torchlight 2: Fantastic RPG, and great with friends.
• Tombraider Guardian of Light: Probably the best local and online co-op with friends. Requires coordination, in a good way.
• Portal 2: Awesome Puzzle Platforming.
• Jamestown: Good bullet hell shooter.
• Castle Crashers: Great beat-em-up and great fun co-op up to 4 players.

If your into it, a couple brilliant old-school Genesis titles available on Steam:

• Gunstar Heroes: Truly one of the best Platforming/shooters every created
• Streets of Rage 1/2/3: Amazing Beat-em-up series. SoR2 is still my favorite game to play with my brother.

Dark Souls! It has its limits tough. But you can coop through most of game.

SpeedRunners, has local coop and still in beta but it's a lot of fun with friends.

Portal 2