Enter either of the following codes at the "Cheats" sub-menu, which is found at the main screen by going to "Extras" > "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:

CHEAT OPTION - DESCRIPTION
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.