Hit tilde (~) in during gameplay to get into the cheat console.

Code              Result
please fly        Free Flight  
please ghost      No Clipping  
please giveall    Get All Items  
please god        Invincibility  
please open       Open All Doors  
please invisible  Invisibility  
please refresh    Refreshes Health  
please killall    Kills all enemies visible on screen  
please tellall    Gives you all messages and intelligence data 
 

Thanks to Revolution reader The Son of!