Hit enter to bring up the chat console and type:

*ghost = noclip toggle 
*cheat = unlimited health and ammo toggle 
*other = brain switch 
*state = ai state 
*suicide = suicide 
*fps = frame rate counter 
*maxfps = toggle max fps between 30 and 60 

Note: Also hitting F8 brings up an undocumented "map mode" and there's 
a secret button in the lower left corner of the main menu screen which 
takes you to practice mode for all missions.