Update: I was unfortunately not aware of Shamus Young's severe criticism of Fallout 3 available here to link in the original piece and I regret that. It dovetails rather nicely with what I've written and it's much better executed than my piece. I strongly recommend anyone...
These codes are entered at the pause screen which can be accessed
Bighead - Gives clones and droids big heads
Darkforce - Gives Sith more strength
Clonehelmet - Gives your charater a clone helmet
Greensaber - Turns lightsabers green