All races:
Enter MATTL66 as a code.
All cars:
Enter YAYCARS as a code.

All tracks and mini-games:
Enter IF900HP as a code.

Maters Speedy Circuit and Maters Countdown cleanup:
Enter TRGTEXC as a code.

Super fast start:
Enter IMSPEED as a code.

Unlimited boost:
Enter VROOOOM as a code.

All paint jobs:
Enter R4MONE as a code.

All movies:
Enter WATCHIT as a code.

All concept art:
Enter CONC3PT as a code.