RPG - Mass effect series. If you like old school, open world dungeons and dragons type stuff, Elder Scrolls: oblivion is fun, and if you can find Skyrim cheap, you will have 6 months of your life gone. IF you like post apocalyptic stuff, the fallout games are fun
Action RPGish games - Red Dead Redemption is fun, Borderlands is fun.
Action - Batman Arkham Asylum is supposed to be good, as is the new one
Figthing games - Get Soul Caliber 4, its still fun. Virtua figher 5 can be found for under ten bucks, Tekken 6 is cheap. Get MK9, best figthing game ever, and SSFIV is great too.
If you like driving/racing games, get Forza IV, hands down the best racing game. ever. EVER