Level Select:
At the Main Menu, press R2, L2, Up, Down, , , .

Level Complete:
During gameplay, press R2, L2, Up, Down, , L3, , , , ,

God mode:
During gameplay, press R2, L2, Up, Down, , R2, L2, R1, L1.

All weapons:
During gameplay, press R2, L2, Up, Down, , Up, , .

Megaforce mode:
During gameplay, press R2, L2, Up, Down, , R2, R2.

Full heal:
During gameplay, press R2, L2, Up, Down, , Up, Down.

Bomb mode:
During gameplay, press R2, L2, Up, Down, , Up, L1.

Punch mode:
During gameplay, press R2, L2, Up, Down, , Up, Up.

Toggle Gravity:
During gameplay, press R2, L2, Up, Down, , L2, L2.

Lethal charge:
During gameplay, press R2, L2, Up, Down, , R1, R1.

Nailgun mode:
During gameplay, press R2, L2, Up, Down, , L1, L1.

Slow motion:
During gameplay, press R2, L2, Up, Down, , Up, L2.

Thanks to Revolution readers Jason, Greg Edge, tony and Simon Mellor!