Mega-Cheat:

Press and hold W+A+X keys while playing. You will get all 9 Weapons, Full Life, & Proximity Mines. However, your score will go to 0 every time you use the cheat.

Cheat Mode:

Start the game by typing C7 LEVEL1DIAGNOSTIC. Now you can use these Cheats…

Backspace+G = God Mode.
Backspace+W = Level warp.
Backspace+N = No clipping.
Backspace+R = Puts a skull in front of character.
Backspace+I = Increases score.

Thanks to Revolution reader Chris!