Secret weapons:
Type baabaa at the weapons options screen. Type aabaab to disable this code.

Unlimited Sheep:
Type bobjob at the weapons options screen.

Super Uzi:
Press [Command] to display the weapons. Then, hold [F1] + [Space] while clicking on the Uzi. Keep the keys held until bazooka shells appear from the Uzi.

Super Mini-gun:
Press [Command] to display the weapons. Then, hold [F3] + [F6] + [Space] while clicking on the Mini-gun.

Level select:
Press [Enter] at the start of a level.