If you enjoy fun racing games then Burnout: Paradise is amazing, you can have hours of fun online just driving around smashing each other up
It's more of an arcade-y racer and definitely not a hardcore simulator (like Shift or Forza).
If you're into JRPGs then get Lost Oddyssey, best one this gen (so far).
Like djdanny said you should definitely get COD4/MW2.
Not sure if you like RPGs much but Mass Effect 1 was absolutely epic! (ME2 comes out next year too)