White Knight Chronicles has an online mode but I haven't played the game yet. Dark/Demon's Souls also do but you can't easily choose your coop partner. Lord of the Rings is an ok game. DCUO is sort of an action RPG and is free. I can't think of any others I've played/own.
I enjoy playing Couch COOP with my family. I'm the father of 6 and the best gamer in the house despite what my 20 year old son believes. I search for good COOP games because no one likes to play me head to head. Additionally, as the father of 6, I need to be able to drop the controller and tend to issues.
The PSN store deos a TERRIBLE job of accurately listing the players and generally doesn't describe the type of multiplayer play.
As an example search for PS3 2 player RPGs and only Borderlands II comes up. Borderlands does not. Open up the game page for Borderlands and players 1-2 the description says split screen COOP.
Additional filter suggestion: make it 3 state: include, neutral and exclude. When I deselect sports, nothing is removed because all of them are also considered strategy or simulation.
Sorry for the rantish nature, but I love the PS and I want to see the store improve.
Borderlands 1 and 2
Resident Evil 5 and 6
Dark Souls and Demon's Souls MAYBE, depends if your up to the challenge ^-^ their's a huge learning curve
Little Big Planet ( 1 and 2)
Dead Space 3 (Just Released)
Far Cry 3
.....I may have left some other co-op games out but the games I listed are all games I had alot of fun and enjoyment out of 'co-op' wise ^_^
These are the games I have extensively played:
Untold Legends: Dark Kingdom - easy hack and slash...online and couch co-op...no trophies
Borderlands - shooter/rpg...very fun...up to 4 online and 2 split-screen
Borderlands 2 - same as first
Demon's Souls - limited online co-op, no couch co-op, challenging gameplay
Dark Souls - Nearly identical to Demon's Souls
Eternal Sonata - JRPG, I think only co-op during battles...no trophies
Dungeon Hunter: Alliance - PSN game, hack and slash
Unfortunately, there are not many quality RPGs for PS3. There are even fewer that have co-op during the regular campaign (not just a co-op mode like mass effect). I know White Knight Chronicles has some kind of co-op play, but I have yet to delve into it fully.