Open New Stuff:
At the main menu, highlight the vehicle select and hold the X + Y buttons simultaneously. While holding these press Up, Left, Down, Right, Right, Up, Down, Right. You will hear a sound when the trick is entered correctly. Now access the vehicle and track select options to see all the new tripods, classes, and tracks at your disposal.

Power-ups are Triple Missiles:
Enter 'missi' as a code.

Power-ups are Turbo Rams:
Enter 'turbo' as a code.

Turbo Ram Auto-Pilot:
Enter 'autom' as a code.

Invisible Tripods:
Enter 'ghost' as a code.

CPU Controlled Bonus Tripods:
Enter 'hund' as a code.

Alternate Sound Effects:
Enter 'retro' as a code.

Debug Mode:
Enter 'johnm' as a code. All items and weapons will be named "Malkovich".

Thanks to Revolution reader Mike Lemense!