My recommendations:
Resident Evil 5: Gold Edition
-It's RE5, with all current DLC, for like $50. Not a bad deal. Inversely, you could try getting a used RE5 for $20, and buy all the DLC for like $10-$15.
White Knight Chronicles
-This is the big one. It's basically a mix between FFXI/FFXII and Monster Hunter.. lots of FF/RPG style free roaming.. no wipe to battle, it's all done on the field.. customizable player character(all the main storyline characters look the same, you customize your own character that's a background character in storymode, but is the only character you play as when participating in the 1-4 player online missions). Next, online missions.
Rock Band 2
-It's expensive to get the bundle, but the game is awesome.. and there's tons of great DLC songs.
Grand Theft Auto 4
-They just released the 2 DLC expansions on PS3.. therefore, PS3 users have a reason to play this again.
Star Wars: The Force Unleashed
-Best Star Wars thing we have for PS3, until The Force Unleashed 2.
Those are the top choices I'd suggest for buying.
Alot of other games suggested, might be good rentals.. particularly Naruto games. Ultimate Ninja Storm wasn't that great, the last 1-2 PS2 titles were better.
That's all for now.