Type "`" when you play the game. 

GOD - Invincibility toggled on or off.
FLY - Up to the rafters! (toggle)
NOCLIP - Go through walls
NOTARGET - Monsters won't attack you unless provoked
GIVE S # - Gives you # Shells
GIVE N # - Gives you # Nails
GIVE R # - Gives you # Rockets
GIVE C # - Gives you # Cells
GIVE H # - Gives you Health # 
GIVE # - Gives you weapon #
IMPULSE 9 - All Weapons
IMPULSE 11 - Get a Rune
IMPULSE 255 - Quad Damage
R_FULLBRIGHT 1 - No more shadows