Press "~" to open the console and type any of the following commands:

 horsey        Activate Cheat Codes

 god           God Mode
 teamgod       Team God Mode
 amphibious    Breathe Underwater
 fly           Fly Mode
 ghost         No Clipping
 loaded        All Weapons
 invisibile    Invisibility
 killpawns     Kill Computer Players
 killenemy     Kill All Enemies
 killcars      Explode All Cars