Press Enter during gameplay and type in the following codes:

Code
<cheat> mpcanthurtme
<cheat> mpschuckit
<cheat> mpbunker
<cheat> mpsmithy
<cheat> mpkohler
<cheat> mpstockpile
<cheat> mpdoctordoctor 100
<cheat> mpbeamme
<cheat> mpsixthsense
<cheat> mpmillertime
 Effect
God mode
All weapons and ammo
Infinite ammunition
Gives armor
Give ammo
Give ammo
Set health to 100
Teleport to start of level
No cliping mode
Marines have slurred voice