Bethesda has released their Fallout 3 editing program, Garden of Eden Creation Kit (GECK), for download.
The editor, first announced a couple of weeks ago, will allow PC owners of Fallout 3 to "expand the game any way they wish. Users can create, modify, and edit any data for use with Fallout 3, from building landscapes, towns, and locations to writing dialogue, creating characters, weapons, creatures, and more."