Hold down F1 while playing the game and type...(cont holding F1 while typing the code)

mukor - God mode
qsave - Quick Save
pause - Pause the game
frame - Screen Shot
golrg - Makes you real big ( try typing this a lot ! )
btiny - Shrinks You
tough - Difficulty Level
mecam - 1st Person View
gocam - Stationary Cam
spcam - Enemy Cam
bamff - Sends you to the end level
gamma - Changes Gamma Level
dedly - Weapon Enhancer
sepku - Kills you

Fun:

bills - Bill's demo type thing
ifall - Knocks you off you feet!!
hicup - Throws you around!!
silky - Stupid Monsters
lunar - Low Gravity
funky - Funky keys toggle
catch - Out of World Trapping Toggle
aiaim - Ai toggle
agrav - Limb gravity

Debug:

bzone - WireFrame/Shading/Nothing/Normal
fpers - Shows "Sound Cache Misses"
goura - Gourad shading
ntrud - Selects An Enemy
plane - Shows Protection And Damage Points
nfade - Disable/Enable Pallate fading
gmode - Faster cycles ( makes everything go faster )
colid - Shows possible collision locations

you can add -god when running the main exe for god mode also.