Prey is another good budget FPS, but it's also available on PC.
Saints Row and Crackdown like MSB said, are both really fun.
RB6:Vegas has a great multiplayer IMO
For XBLA: Geometry Wars and CastleVania:SoTN are both great.
Currently Playing: Saints Row 2 | Battlefield 2 | Company of Heroes
Recently Beaten: Gears of War | Super Mario Galaxy | Darwinia | MGS4 | Sam and Max Ep. 4, Portal | Mirror's Edge | Uncharted | Mass Effect
Looking Forward to: Alan Wake | Splinter Cell : Conviction | The Last Guardian | Batman: Arkham Asylum | SMG2 | Mass Effect 2 |