Enter the following in the Cheat menu:
YAYCARS - All Cars
IMSPEED - Fast Start
VROOOOM - Infinite Boost
R4MONE - ???
TRGTEXC - Unlock Master's Speedy Circiut and Master's Countdown Cleanup
MATTL66 - Unlock all tracks and mini games
IF900HP - Unlock all tracks and mini games
CONC3PT - Unlock Art
WATCHIT - Unlock Movie Clips