My most recommended PSN Games:
Trine is great in single player and in couch coop. It's a platformer where you control three fantasy characters -- an armored warrior, a female thief with a box and a grappling hook, and a wizard who can magically summon boxes and platforms. Coop is hilarious and actually more challenging than single player, because in single player you can change characters whenever you want. In coop, you have to figure out how to get everyone through every puzzle, so the game changes significantly. It's also a pretty long game for a PSN title -- it took me about 7 hours.
Flower is just a really great experience. Even friends who only play shooters have beaten this game and loved it, and now have to deal with the fact that they love a game about flowers and the wind. It starts out as the most relaxing game I've ever played, and from there it gets interesting.
Fat Princess is a really unique online multiplayer game, and underrated on metacritic because of some serious launch issues.
Pixeljunk Eden if you like simple, challenging gameplay and couch coop. It's like playing a retro game that never quite existed.
Shatter is a breakout game slash shmup, kind of... ok, not really a shmup but it has boss battles and energy shields and magnetism and manages to feel like it's not a breakout game either, but something new. Easy to play through in one sitting without realizing it, which takes a decent chunk of time.
Since you don't have a 360,
Castle Crashers and Braid (which is also on PC) when they come out later this year. (They came out on 360 first, and are two of my favorite games in years.)
Hope this gives you some ideas.
