Theres a few tricks you can do, and several hidden places most people never find, but that's all the cheat codes I've found. It's not like the old Doom, where you could enhance your character, add ammo, whenever you wanted.
I went to the Halo site, read the official stuff. There's links to other places, and on a couple of them a private citizen or two will explain a few tricks. But I never used them, they were pretty insignificant.
munk