During play, press ~ to bring down the console. Now enter one of the following codes and press Enter to activate.

toggle g_p_god   God mode                
give all         All weapons and items   
give ammo        All weapons            
give jetpack     Jet pack                
give forcefield  Force field             
give life        Extra life            
give nuke        10 nukes                
give health      100 ego                
give secret      Mark secrets       
give bomb        Spawn extra babe        
pause            Matrix-style pause     
kill             Commit suicide       

Alternately, bring down the console, type exec cheats.cfg and press

Enter. Close the console. Now during play, press the following keys to activate

the cheats:

G    Gives everything except jetpack and force field
H    Gives all ammo
J    Gives jetpack
F    Gives force field
L    Gives extra-life
K    Kill self
I    Turns invincibility ON/OFF
P    Developer's pause ("Matrix" mode)
-    Small HUD
=    Normal HUD
M    Shows map name you are on 

Thanks to Revolution readers Link, Big Z, and Grinder!