Enable Console:

Select Video/Audio Menu --> Advanced Options
--> Checkmark Console. Once playing the game,
bring down the console by pressing ~. Enter 
any of the following codes at the console:
 

Code:      Result:
god        God Mode
notarget   No Target Mode (Enemies Don't Attack)
noclip     No Clipping Mode (Walk Through Walls)
give all   All Weapons, Items
health 100 Full Health
eventlist  Show All Possible Commands
map #      Switch to Map # (See List Below)
 

Map Listing 

blood
cemetary
cliff1
cliff2
creeperpens
end
fakkhouse
fog
gruff
gruff_cinema
homes1
homes2evil
homes2good
intro
landersroost
oracle
oracleway
otto
over
shield
swamp1
swamp2
swamp3
towncenter_evil
towncenter_good
training
under
water
zoo


by unknown