Various cheats
     During  gameplay,  press  [~] for the console and type any of the
following codes:

Cheat code:             Effect:
please god              God Mode
please giveall          Give All Weapons / Ammo
please killall          Kill All Monster On Level
please open             Open All Doors
please fly              Fly Mode
please ghost            No Clipping Mode
please invisible        Invisibility
please tellall          View All Messages
please refresh          Restore Energy

Enable Developer Cheats 
     Hit  the tilde key while playing the game to bring up the console
and type "sam_iEnableCheats=2" without quotes. Then pause the game and
hit  F1 and you will have enabled the developer cheats rather than the
regular ones.

Cheat code:             Effect:
Turbo Option            Makes Sam Incredibly fast and Jump Far
Chapter Option          Warps you around current level
God Mode Option         Sam will take no damage and will 
                        not be knocked back.
Kill All Option         Destroy all enemies simultaneously
Refresh Heath Option    Bring Health to 200%
Add Life Option         Adds one additional life to however many you have
Toggle Hud Option       Turns the Heads Up Display On/Off
Ghost Option            Allows Sam to fly
Invisibility Option     Sam's Enemies will no longer see him
Unlock Levels Option    Unlocks all levels in the Custom Level Menu
Level Skip              After you do the F3, F4, F3, F3, F4 cheat 
                        at the main menu, when you are in a level, 
                        you can press F2 (NOT IN THE CHEAT MENU) 
                        to skip to the next level. 
                        This works in any level but the last one.


Cheat Mode:
     Hold  [Left  Shift],  then  press [F3] [F4] [F3] [F3] [F4] at the
main menu. Then, press [F1] to display the cheat menu.

Cheat Mode (Alternate):
     Start the game as usual. Press ` (tilde) to bring up the console.
Type sam_iEnableCheats=1 and press [Enter]. Close the console (press `
again)  and  then bring up the main menu by pressing [ESC]. Access the
cheat menu by pressing [F1].

You can also disable the cheats with:

sam_iEnableCheats=0