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 damage        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.