To enable cheats, type t, then delete 'Say' with the backspace.  Now type in:
set derzeitmann.kevin bcheatsenabled true
press enter and cheats are on.  Here are some useful cheats:
God - invincibility
ghost - go through walls, floors, doors, etc.
fly - fly (not through walls floors, or doors)
walk - brings you out of ghost or fly mode
allammo - gives you all ammo for the weapons you have
summon derzeitmann.ITEMNAME - creats the itemname infront of you,
Where ITEMNAME is:
weaponpulser
weaponpulserassault
weaponpulserShotgun
weaponpulerrifle
weaponplasmainvisible
ellitesoldat
humansoldat
humansoldatcommander
humancommander
humanercommando
council1
council2
navalofficer
navalcommander
dornas
nevark
lorak
ammolightpulser
ammomediumpulser
ammoheavypulser
ammoplasmablast
...and so on and so forth.
If you go into the credits, you can also type these for neat effect (they need not be capitilized):
DORNAS
DAISTKEINSPOON
SCHRANK
GROSESKOPF
IAMWARREN
MEHRFROSCH
But you have to have cheats on to do those (just a reminder).