Commando

At the title screen, press Left, Right, A, B, A, Down, abd Up on Player 2's controller, then hold A on Player 1's controller By pressing the Select Button, you can select the desired level.

If you can finish Mission 4-4 with 3 lives remaining and 24 grenades remaining, you get a secret message from the programmer after the usual ending message.

At title screen, press B, Down, Up, A, Left, Left and Right on the 2nd controller. Now, during the game, press or hold Select to move frame by frame.