Secret Chainsaw weapon
Pause the game and press Up, Down, Up, Down, Left, Right, Left, Right, Y, X, Y, X.

Secret Baby Rattle weapon
Successfully complete the game on the Easy difficulty setting. Then, pause the game and press Left(2), Right(2), Y, X, X, Y, Up, Down.

Secret Telephone weapon
Successfully complete the game on the Normal difficulty setting. Then, pause the game and press Right, Left, Right, Left, Down(2), Up(2), Y, X, Y(2), X(2).

Secret Swordfish weapon
Successfully complete the game on the Hard difficulty setting. Then, pause the game and press Up(2), Down(2), Left, Right, Left, Right, Y, X, Y, X.