Good co op games for long term?

What are some games that I can play with my friend for a long term? no wow or swtor been there

My specs are:

Intel(R) Core(TM)2 Quad CPU Q8300 @ 2.50GHz
4 gigs of ram
ATI Radeon HD 5570

Hes specs are a tad better was thinking about arma as I think there are many co op missions in that but I dunno if I can run it?

Left 4 Dead 2 is the best co-op game you can play. It requires proper teamwork and planning. It also just happens to be a great game. Endlessly replayable and some of the best combat in any game. Portal 2 is second. Fantastic co-op interaction but obviously its not as replayable. Stay away from BF3's co-op. It was actually broken the last time I tried it. biggest_loser

Diablo 3 in 10 days 6 hours and 40 minutes, forgot another great one, Dungeon Defenders and you're in luck, check indieroyale.com, however, it does have a lot of DLC that you can get for cheap on a sale, but that might discourage you (guys).

I recommend all the games recommended by the posters above me and the following:

Serious Sam 1, 2 and 3

SWAT 4 plus the expansionSWAT 4: The Stetchkov Syndicate (gem)

Magicka

Diablo 3 (May 15th) and Torchlight 2 (anticipated in a couple of months)

Diablo 3 shouldn't be a problem on your system.

Mass Effect 3, Dead Island, HL2 has a coop mod, Jamestown has local coop, Portal 2, Saints Row 3, Sanctum, Trine 2, and Dawn of War 2 series.

Payday the heist

guildwars

C&C: red alert 3

supreme commander 2