While playing a game, press T to display the console window. Then, enter one of 
the following codes to activate the corresponding cheat function:

Result - Cheat Code
Level skip - retreat 
God mode - johnwayne 
All weapons, full health and armor - almighty 
All weapons - onemanarmy 
Full ammunition - getsome 
Full armor - fullmetaljacket 
Full health - didnthurt 
Infinite ammunition - rocknroll 
Spectator mode - ghost