XBLA:
Aegis Wing *free* (co-op)
Bionic Commando Rearmed (co-op)
Bomberman Live
Castle Crashers (co-op)
Earthworm Jim (co-op)
Geometry Wars Evolved 2 (co-op)
Ikaruga (co-op)
Ilomilo (co-op)
Lara Croft: GoL (co-op)
Peggle
Splosion Man (co-op)
Retail:
Blur
Command and Conquer 3
Command and Conquer 3: Kane's Wrath
Command and Conquer Red Alert 3 (co-op)
Family Game Night *try the demos as some of them are good*
Forza
Guitar Hero *Any of them* (co-op)
Halo Wars (co-op)
Legos *Any of them* (co-op)
Marvel Ultimate Alliance (co-op)
Mortal Kombat
Need for Speed Hot Pursuit
Portal 2 (co-op)
Rockband *Any of them* (co-op)
Scene It?
Simpsons, The (co-op)
Soul Calibur IV
Street Fighter
Trivial Pursuit
You Don't Know Jack
Coming Soon:
Dungeon Siege 3 (co-op)
Just from the games I've played.