For the past 5 years I've only had a Wii. I finally got fed up with missing all the cool games and bought a PS3 and some games from a co-worker a few weeks ago.

Right now the only games I have are Mafia 2, Skate 3, and Katamari Forever. I'm looking for recommendations of some "essential" games to play. I'll admit I'm almost entirely clueless on what to buy. Preferably I'd like games that have been out for a few years, that way I can buy them used for cheap. I'm pretty much open towards every genre except sports.