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.