Green Gas Fart:
While playing, hold R1 then press Up, Left, Right, Square, Circle, X. Make sure you're wearing your gas mask!

Level Select:
At the main menu hold R1 & press Down, Right, Left, Right, Square, Circle, Square, Triangle, Circle, Square, Right, Left.

Level Skip:
At the options screen hold R1 & press Up, Right, Left, Square, Circle, Triangle, Square, Right, Left, Up, Right.

Show All FMV's:
At the main menu, hold R1 & press Up, Left, Right, Square, Circle, Triangle, Square, Right, Left, Up, Right.

Voice Cheat:
At any time without pausing the game, hold R1 & press Triangle, Up, Circle, Left, X, Down, Square, Right. You can now solve any voice puzzle with any sound.