All of these can be activated in the Extra's menu. Seeya at Cheating Dome!

Unlockable	Task
Duke 3D Freeze Ray	Complete the game.
Grayscale Mode	Complete the game.
Head Scale	Complete the game.
Infinite Ammo	Complete the game.
Instagib	Complete the game.
Invincibility	Complete the game.
Mirror Mode	Complete the game.