CheatOgre.com: Jeremy McGrath Supercross World

Moon gravity:
Press Left, Right, Up, Down, B(3) at the main menu. The lower left corner of the screen will flash to confirm correct code entry.

Unlimited turbo:
Press Down(3), L, R, Z at the main menu. The lower left corner of the screen will flash to confirm correct code entry.

All bikes:
Successfully complete the hardcore part of career mode.