Enter these codes on the name wheel. 

CARNIVAL        All cars 
CHANGELING      Change car mid-race 
DRINKME         Small cars
JOKER           Opponents cars look like your model in multiplayer 
MAKEITGOOD      Enables in-game editors 
SADIST          Select weapons in game with Right Shift 
TRACKER         Unlock all tracks 
TVTIME          Enables F6 and F5 cameras in game 
URCO            UFO selectable