Bring up the console (press ~) and enter the following codes:

  god
                    God Mode
  notarget
                    No Target Mode
  noclip
                    No Clipping
  give all
                    All items, weapons, 999 ammo
  give jacketarmor
                    Jacket Armor
  give blaster
                    Blaster
  give shotgun
                    Shotgun
  give sshotgun
                    Super Shotgun
  give machinegun
                    Machine gun
  give grenadelauncher
                    Grenade Launcher
  give rocketlauncher
                    Rocket Launcher
  give shells
                    Shells
  give bullets
                    Bullets
  give cells
                    Cells
  give grenades
                    Grenades
  give rockets
                    Rockets
  give slugs
                    Slugs
  give quad
                    Quad Damage
  give invulnerability
                    Invulnerability
  give silencer
                    Silencer
  give rebreather
                    Rebreather
  give health
                    100% health
  give weapons
                    All weapons
  give ammo
                    All ammo
  give armor
                    Body armor
 

Noticing a pattern? That's because you can type just about anything after 'give' and you'll get it....Try it with any other weapon or
item and see!

To skip levels:
Press ~ to access the console, then type map xx where x is the level name. To get level name go to multiplay, host game, and
select level name you wish to skip to (the bottom name is the name of the level). For instance, ~ map boss2 will take you to the
final level.

Gordon Rice, Anarchy743, John Doe Jr., Christian, Paul Yazhemsky, Mike, Steve, Kevin
Shultz, Gary Unwin, and Derek Jones!