Bring up the console and type in:

 +6dof             1st person view
 BIGFINALE         To the end
 DIR MAPS          Lists all maps
 MAP < Mapname >   Loads a map
 FLY               Fly mode
 NO CLIP           No clipping mode
 ODE TO JACK       Kill all enemies
 IMPULSE 10        Magical attack
 IMPULSE 11        God mode