Cheat mode:
Select the "Settings" option at the main menu, then select the "Cheat Codes" option:

Result - Code:

God mode - F3ARM3
All ammunition - ALLAMO
Cinematics can now be skipped - P33kaB00
Exclusive Best Buy multi-player level - BESTBUYpwnsYou
Exclusive EB Games multi-player level - Ebpwnz
Exclusive Future Shop multi-player level - FuturePwnage
Exclusive Gamestop multi-player level - GameStopPwnage
Exclusive Groove multi-player level - YouGotGroove
Exclusive Target multi-player level - TargetingYou
Exclusive Toys R Us multi-player level - TRUPwned
Exclusive Walmart muti-player level - PwnageMart
Secret content unlocked - ShhItsaSecret
Single player levels unlocked - IMALAZYBEEOTCH
Toggle location stats - MYLOCATION

Cheat mode (alternate):
Press ~ during game play in single player mode to display the console window:

Result - Code:

God mode - god
No clipping mode - ghost
Flight mode - fly
999 ammunition for all weapons - allammo
Set field of view; default is 70 - fov
First person view - behindview 0
Third person view - behindview 1
Suicide - suicide
Bots in multi-player game - addbots
Spawn WEC; for upgrading weapons summon - VehicleGame.WeaponEnergyCore