After completing the game you can enable the cheat mode. During the game you can hit the F1 button and enter:

LIVEFOREVER	god mode
SHAPESHIFTER	change characters with F8
TOUCHOFDEATH	Omnipotence
CANTTOUCHTHIS	unstoppable
FATLOOT	ammo + health
GLASSWORLD	glass furniture
WINLEVEL	win level
BIGHEAD	big heads!
MINIME	mini mode
SUPERAMMO	super ammo mode
RESERVOIRDOGS	Bad guys fight with each other
ROUGHJUSTICE	gatling guns mode!
CHENILLE	daodan power mode
BEHEMOTH	GodzillaTM mode
ELDERRUNE	regeneration
MOONSHADOW	phase cloak
MUNITIONFRENZY	weapons locer created
FISTSOFLEDGEND	Fists of Legend mode
KILLMEQUICK	Ultra mode
CAROUSEL	slow motion
THEDAYISMINE	Developer mode!

Once you activate Developer Mode (using "thedayismine" code), press the buttons below to get the desired effect:

F8: Cycle through your all weapons
F9: Begin recording
F10: Stop recording
F11: Play back what you've recorded
Control-Shift-B: Display your opponent's logic
Control-Shift-G: Toggle slo-mo on/off
Control-Shisft-S: Toggle textures
Control-Shift-Y: Display current frame rate
Type "ai2kill": Kill all nearby enemies
Type "chr_nocollision1": Toggle no clipping mode on
Type "chrnocollision0": Toggle no clipping mode off
Type "dump_docs": Display console commands