Enable the console by going to Video/Audio, then Advanced options, then click console. Now during gameplay, enter any of the following:
Code
Effect
god
God Mode
give all
All Weapons
map (mapname)
Jump to mapname (below)
notarget
Enemies Don't Attack
noclip
No Clipping
health 100
Full Health
eventlist
Show All Possible Commands
Map Names
Use with map (mapname):
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