I highly recomend to buy pre-owned games at Gamestop and test them out if you like the game then keep it or buy a new copy.(Some games can not be played in multi player unless you have an online pass for them)
For first person shooters: Halo Reach, Modern Warfare 2 or 3, Homefront, Battlefield 2 bad company.
For rpgs: Fallout 3, Mass Effect 1-3(third one requires online pass for online play) Resonance of Fate, Magna Carta 2, Lost Odyssey, Star Ocean:The Last Hope, Dragon Age, Dragon's Dogma.
Figthing:Ultimate Marvel vs Capcom 3, Dead or Alive 5, King of Fighters.
Misc: Dance Dance Revolution Universe 1-3, Guitar Hero, Rock Band, Magic 2013, The Walking Dead.
I can keep going but it will take too long.