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 

Thanks to Revolution reader Martin Brodeur!