Enter these codes on the name wheel.

carnival ... All cars
changeling ... Change car mid-race (PGUP or PGDN)
drinkme ... Smaller cars
joker ... Opponents cars look like your model in mutiplayer
makeitgood ... Enables in-game editors
sadist ... Cycle through all weapons (R_SHIFT)
tracker ... Unlock all tracks
tvtime ... enables F6 and F5 cameras in game
urco ... ufo selectable

To unlock Clock Carnage Mode, obtain all the stars in the Stunt Arena Mode

Type in cartoon as your name.