Press T for the "Talk" feature, then enter one of the following codes then:
God mode: mpgod
All weapons: mpkfa
No clipping: mpclip
Full health: mphealthy
View coordinates: mpwhereami
Hide coordinates: mphideme
Kill all monsters on level: mpkillemall
Sniper rifle:mpsniperrifle
Howitzer: mphowitzer
All items: mpgoshopping
Increase speed: mpspeedup [1-5]
Increase strength: mpstronger [1-5]
Invisibility: mptakeoffshoes
Add 25 to health: mpnicenurse
300 health: mpreallynicenurse
Napalm cannon: mpnapalmcannon
Singularity generator: mpsingularity
Assault rifle: mpassaultrifle
Bug buster: mpbugbuster
Minigun: mpminigun
Cobalco laser rifle: mplaserrifle
Tesla cannon: mpteslacannon
Voodoo doll: mpvoodoo
25 to armor: mpward
100 armor: mpnewcroward
Full ammo: mpammo
Bonus weapons: mpbeansofcoolness