Castle Crashers is up to four players. Side scrolling beat-em-up with some RPG elements.
DeathSpank and its sequels/spinoffs are co-op RPGs I think, only seen someone else play it though.
Outside of PSX titles RPGs are somewhat rare on PSN so far as I recall. Co-op ones would be even rarer. There's a good chance I just don't know about some of them though.
As far as just co-op goes, there's a Bomberman title or two up that's definitely co-op, really any Bomberman likely supports it. The most recent was pretty decent. There's some persistent elements tied to your account, but they're purely aesthetic unlocks.
I remember picking up Final Fight: Double Impact awhile back (the "double" is another Capcom side scroller called Magic Sword, has nothing to do with Final Fight.) Both co-op games. Magic Sword has some very minor RPG elements.
In that vein, the vast majority of Capcom arcade games are co-op, but I'm not sure how many are up on PSN without looking. What I do know is up is a ton of Neo Geo games. You'll definitely want to check before you purchase any, but almost all of them were co-op. Kinda expensive
Beyond that... pretty sure there's a Contra game up (Hard Corps Uprising?) that wasn't on a previous console. No idea how good it is though.
Lastly, found a co-op list here:
http://www.co-optimus.com/system.php?id=12&page=1Might help better than my rambling.