Although mentioned elsewhere, heres what we play together (co-op or no):
Cod MW2 spec ops - FPS
Fifa 13 - sports
Van helsing 1,2 (just bought 2 in bundle, so we lowered difficulty in 1 and are just doing story...1 has some unfixed bugs in co op :/)
Insurgency COOP server - FPS
Left for dead 2 - FPS
Alien Swarm (free) -top down shooter
Risk of Rain - 2d platform
Path of Exile (free) - ARPG (diablo type)
Lara Croft and the Guardian of Light (finished main story now tho) - ARPG (sp diablo type?)
Battlefield 3 -FPS
Some we play more often than others, but our "ritual" is first game (and usually last game before he goes) is a fifa
So those are some nice co-op, or at least 2 player type games that are (or were at some point) fairly cheap or even free