To use these codes, just type them in at the console.
(Press the ~ key to get to the console)
GOD             God Mode
NOCLIP          No Clipping Mode
IMPULSE 9       All Weapons and Keys
IMPULSE 255     Quad Damage
GIVE #          Gives any # gun
GIVE R #        Get Rockets
GIVE N #        Get Nails
GIVE H #        Get Health
GIVE S #        Get Shells
GIVE C #        Get Cells
KILL            Suicide
FLY             Fly Mode
MAP E#M#        Level Warp
NOTARGET        Enemies Won't See You