Level Select:
At the Main Menu, press R, L, Up, Down, X, Click Left Analog Stick, B, A, B, A

God mode:
During gameplay, press R, L, Up, Down, A, R, L, Black, White.

All weapons:
During gameplay, press R, L, Up, Down, A, Up, X, A.

Megaforce mode:
During gameplay, press R, L, Up, Down, A, R, R.

Full heal:
During gameplay, press R, L, Up, Down, A, Up, Down.

Bomb mode:
During gameplay, press R, L, Up, Down, A, Up, White.

Punch mode:
During gameplay, press R, L, Up, Down, A, Up, Up.

Toggle Gravity:
During gameplay, press R, L, Up, Down, A, L, L.

Lethal charge:
During gameplay, press R, L, Up, Down, A, Black, White.

Nailgun mode:
During gameplay, press R, L, Up, Down A, White, White.

Slow motion:
During gameplay, press R, L, Up, Down, A, Up, L.

Thanks to Revolution readers Simpson McMurphy, SuRF3R,a123a130 and XXX!