Even though there is no editor released for Fallout, some people have managed to pull it off anyway. As predicted, these mods mainly apply to game mechanics (values changed so chems and stuff have slightly different effects) and texturing (game interface, armour textures, etc) since this is all possible without an editor, as it was in Oblivion as well.

I have uploaded some of the nicer mods for Fallout 3 on the site today, so you can all start downloading to your hearts content. Make sure to read the description or the accompanying readme.txt for installation details. Installing is a little more work since the game does not support mods by default (yet).

Some news is still going around that Bethesda will eventually realease a modding toolkit for us in the end, but I wouldn't put any money on that. It seems as though the modding community is left out in the cold for this one. 

Download Fallout 3 mods here