Cheat mode:
In order to enable these codes you must download the patch. Enter -debug as a command line parameter to enable cheat mode.

Here's an easy way of doing this:
Create a Shortcut to the game on your Desktop. Right click on it, go to properties. You will see something like: c:\****\****\exe. Just add -debug to the end of this command line and then click Apply. Now start the game.

Press ` then enter one of the following case-sensitive codes to activate the corresponding cheat function.
*Note: Enabling cheat mode will disable saving the game.

MOTIONTRACKERVOLUME (0.00-1.00) Change motion tracker volume for a Marine    

MOTIONTRACKERSPEED (0-16)       Change motion tracker speed for a Marine    

GIMME_CHARGE                    Replenish energy supply Predator   

CROUCHMODE (0 or 1)             Toggle crouch and crawl for a Alien   

ALIENBOT                        Create CPU controlled Alien    

GIVEALLWEAPONS                  All weapons and ammo  

GOD                             God mode    

LIGHT                           Create light aura; repeat for brighter aura    

MARINEBOT                       Create an CPU controlled Marine Any     

PREDOBOT                        Create CPU controlled Predator1     

PREDOBOT                        Create CPU controlled Predator1     

PREDALIENBOT                    Create CPU controlled Predator-Alien1    

PRAETORIANBOT                   Create CPU controlled Praetorian Alien1     

XENOBORG                        Create CPU controlled Xenoborg1   

SHOWFPS                         Display frame rate Any  

SHOWPOLYCOUNT                   Display number of rendered polygons Any   

LISTVAR                         List in-game help   

LISTCMD                         List cheat commands   

TIMESCALE (0.0-1.0)             Change game speed   

Hints

Pigsticker:
Finish Level I on Medium Difficutly with 80% or more accuracy, and get the "Pigstick" Cheat. The "Pigstick" cheat releases about 10 spears from the speargun. EXTREMELY effective at close range.

Get the Sipemunch:
Getting 10 civilian live head-bites in alien level 1 will get you snipermunch! You can head-bite from long distences.

Nasty!:
Finish Level II on Medium Difficulty gathering, 25 or more trophies, and get the "SuperGore" Cheat. The "SuperGore" cheat is self-explanitory. It increases gore by about 3 or 4 times.

Infinite Grenades in Co-Op Mode:
Start a Co-Op game on a level of your choice. Choose Marine-Grenade as your class. You have three classes of grenades: Normal, Frag, and Proximity Mine. Shoot off all rounds of your two least favorite types. After all of two types of grenades have been fired off, fire off all of your favorite type of grenade. After you are completely out of grenade ammo, press the button you have set as secondary fire. You should now have a just about infinite amount of your favorite type of grenades.