During game play press the '~' tilde key to bring up the console and then enter any of the following codes to unlock the corresponding effect.

Unlock God Mode, ALL Weapons and 999 Ammo for each Weapon:
All

Unlock Invincibility:
God

Unlock Flying Ability:
Fly

Unlock ALL Weapons:
Weapons

Unlock Fire Axe:
Axe

Unlock Hammer:
Hammer

Unlock Pump-action Shotgun:
Shotgun

Unlock Bolt-action Sniper Sifle:
Sniper

Unlock Machine Gun:
Machinegun

Unlock Revolver (Pistol):
Revolver

Unlock Glock (Pistol):
Glock

Unlock .50cal Gatling Gun:
Minigun

Unlock Kung Fu Fists:
Kungfu

Unlock Molotov Cocktails:
Molotov

Clipping Off:
Ghost

Clipping On:
Walk

Unlock 1st Person View:
Behindview 0

Unlock 3rd Person View:
Behindview 1

Unlock 999 Rounds for the currently equipped Weapon:
Allammo