While playing, bring down the console by holding down [~] + [F12] at the same time.
Then, enter:
 
MARINE Cheats:
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alienbot           - Creates AI controlled alien
freakoftheuniverse - God mode
giveallweapons     - All weapons/ammo
god                - God mode
light              - Creates light around player
marinebot          - Creates AI controlled marine
observer           - Observer mode on/off
predobot           - Creates AI controlled Predator
showcoords         - Shows level coordinates
showfps            - Show Framerate
 
PREDATOR Cheats:
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gimme_charge       - Replenishes Predator energy
skullcollector     - God mode
giveallweapons     - All weapons/ammo
god                - God mode
light              - Creates light around player
marinebot          - Creates AI controlled marine
observer           - Observer mode on/off
predobot           - Creates AI controlled Predator
showcoords         - Shows level coordinates
showfps            - Show Framerate
 

ALIEN Cheats:
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theonedeadlycreautureevercreated - God mode
giveallweapons     - All weapons/ammo
god                - God mode
light              - Creates light around player
marinebot          - Creates AI controlled marine
observer           - Observer mode on/off
predobot           - Creates AI controlled Predator
showcoords         - Shows level coordinates
showfps            - Show Framerate
winneroftheonegreatbattleoftheuniverse - Become the final Xenomorph