Due to both having an unusually bad work schedule this semester, my co-op partner and I have been looking for something a bit less thinky than our usual fare for those nights when we're both too exhausted to think clearly. The ideal candidate for this kind of thing tends to be a co-op FPS, but we've only got a couple of those so it's probably time to expand our selection of options.
Stuff along this line that we've played and enjoyed:
L4D 1 and 2 (up to the point where the bot stupidity becomes too frustrating to bear)
Brothers in Arms: Earned in Blood (wish there were more co-op maps, this one's just great)
Serious Sam HD 1 & 2
Sanctum (though there's actually some real strategizing in this one)
Probably others, but I'm coming up blank. Co-op campaigns are a real plus here. Any recommendations welcome. Thanks!