Master code:
Enter either of the following codes at the "Cheats" sub-menu, which is found at the main screen by going to "Extras", then "Cheats": YZWZYXWX or WWXXYZYZ. This will unlock all in-game cheats. It has to be "activated" by putting a checkmark next to it. You can also access this menu during game play by pressing [Esc], then going to "Cheats". The cheat options are as follows:
Information in this section was contributed by Defender. 

Super Health: Health quickly regenerates over time.
Infinite Grenades: Self-explanatory.
Infinite Ammo: Self-explanatory.
Reign Of Fire: All weapons make the target burst in flames after they are shot.
Explosive Personality: When pressing [Use] (default E) on someone, they gib.
Walking Dead: Makes everyone resemble zombies.
Gibby Ammo: Shoot one round and the victim gibs.
Gibby Explosions: Explosions make everything in its path gib. 
Rapid Rails: RailGun shoots very fast.
Extra Chunky: Every time you shoot an enemy, you get a gib part (only visually with no hole)
Wacky Deaths: When shot in front, some enemies you kill die as if hit hard in the stomach; when shot in back they look like they tripped on something. 

Cheat mode:
Enter one the following codes at the "Cheats" sub-menu, which is found at the main screen by going to "Extras", then "Cheats", to unlock the corresponding cheat option. 

Effect, Code
 Effect, Code
 
Super Health, xxywywz
 Infinite Grenades, zxzyxzxz
 
Director's Cut, yxzwzxyw
 Rapid Rails, zyzyxxww
 
Gibby Ammo, xxxxyzxx
 Extra Chunky, zzzzwxzz
 
Bouncing Bombs, zzzzzzzz
 Infinite Ammo, ywxzyzxw
 
Wacky Deaths, wwwwwwww
 Walking Dead, xxxxxxxx
 
Rain of Fire, yyyyyyyy
 Gibby Explosions, wzxywzxy
 
Explosive Personality, x
 Win Game, yxyx
 
All Weapons, y
 Unlock Everything, wwxxyzyz
 
Unlock All Cheats, yzwzyxwx
 Unlock All Levels, zyxwyzxx
 
Super Health, xxywywz
 Infinite Grenades, zxzyxzxz
 
Director's Cut, yxzwzxyw
 Rapid Rails, zyzyxxww
 
Gibby Ammo, xxxxyzxx
 Extra Chunky, zzzzwxzz
 
Bouncing Bombs, zzzzzzzz
 Infinite Ammo, ywxzyzxw
 


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Hint: FMV sequences:
After clearing a level, you will be awarded the FMV sequence for that level plus another movie. Completing level 1 unlocks a spoof interview with characters from Summoner 2 talking about the Red Faction games. Completing level 2 unlocks a behind-the-scenes FMV sequence.