Use ` to bring up the console window and use the codes from the list:

Code                     Result 
please god               God Mode 
please giveall           Give All Items 
please killall           Kill All Enemies 
please open              Open All Doors 
please fly               Fly Mode 
please ghost             No Clipping 
please invisible         Invisibility 
please tellall           Show All Messages from Netricsa 
please refresh           Resets Health to 100