Hit enter to bring up the chat console and type:

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

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