While ingame press TAB to activate the console and enter the 
following:

Code                 Result
---------------------------
God                - God mode
BadAss             - All skills at max 
Invincible         - One hit kills
Useammo            - Unlimited ammo
Teleport           - Teleport to the location of the crosshair
Suicide            - Kill yourself
PlayersOnly        - Freeze all objects
Blizzow 1          - Set Weapons and Power to max
Blaster            - Gives H.A.Z.E weapon
Pistol             - Gives Pistol
XJ9                - Gives XJ9 weapon
Launcher           - Gives Talmage weapon
SeekerPistol       - Gives Talon weapon
PulseGun           - Gives Acolyte weapon
SeekerXJ9          - Gives Darkfire Weapon
SeekerMicrowaveGun - Gives Discord weapon
Assault            - Gives Kaull weapon
SeekerLauncher     - Gives Fury weapon
Photon             - Gives Stalker weapon
StunGun            - Gives Stungun weapon