While playing a game, press [F1] or ~ to display the console window. Then, enter 
one of the following codes to activate the corresponding cheat function:

Result - Cheat Code
God mode - please god 
All items - please giveall 
Kill all enemies - please killall 
Open all doors - please open 
Flight mode - please fly 
No clipping mode - please ghost 
Invisibility - please invisible 
Unknown - please tellall 
Unknown - please refresh