Taken from a post I made in my gaming group of friends on Facebook.
Current co-op/group games available or that we are already playing:
Planetside 2
Dota 2
Lotr: War in the North
Din's Curse
Borderland's 2
Torchlight 2
Castle Crashers
Total War: Shogun 2
Dawn of War II: All expansions
Games we haven't tried out but might be worth looking into:
Path of Exile (like Diablo, closed beta, but I just started it up and I hear good things)
Farcry 3 (can't co-op whole game, but apparently has fun co-op missions?)
Cry of Fear
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We recently added Path of Exile to our playlist and its been a good amount of fun so far. Also a friend picked up the Ace of Spades 4 pack and we'll probably play that sometime this weekend.