While in game press "Enter" and type " " (include the space after ), followed with the one of the codes from below and then press "Enter" to enable the code.

Code - Result
mpsixthsense - No Clipping
mpicu - Third Person View
mptachometer - Show Speed Info
mpsizeme - Show Size Info
mpcanthurtme - God Mode
mpschuckit - Add Weapons & Ammo
mpsmithy - Full Armor
mpkohler - Full Ammo
mpbeamme - Beam Player to Level Start
mpmorph - [character] Change to [Character]