Uncharted, Resistance, and inFamous are some of the better ones.
Get Borderlands, its great fun by yourself and with friends. Has splitscreen as well.
Starhawk is pretty awesome. It feels like it has kinda a steep learning curve at first, but actually after about a dozen rounds you start surprising yourself with epic build+battle combos and strategy. I also highly suggest killzone 3, party because I am a huge sci fi geek and it is probably the best sci-fi game I have ever played, but also because holy stuff its really epic, just like god of war 3 and such.
PSN is Cdog1st
add Feep13, sordykins
Battlefield 3 and Just Cause 2MagickDood