Topic needs more LittleBigPlanet, the quintessential PS3 co-op game. There's also Demon's Souls, which has an online buddy system to help you through the usually solitary campaign, and Resistance: Fall of Man for some couch co-op action.
On the PSN side of things, I really enjoyed playing though Pixeljunk Shooter and Eden with my room mate, as well as Super Stardust HD.