Bethesda has released
the GECK editor which will allow you to create mods, levels, and generally play with everything in Fallout 3
. In conjunction with the release of the tool Bethesda has published a Wiki
that shows you how to use the engine as well as some videos
showing the tool in action.
This is the point where people need to be reminded that with great power comes great responsibility and that we really don't need a bunch of crappy nude modes for the game or the addition of giant penis shaped monsters in the game. I know this cry will fall on deaf ears but a guy can hope can't he?