Bethesda’s released the Garden of Eden Creation Kit (GECK) for Fallout 3. Download it here.
As he company would have it, “The GECK provides the community with tools that will allow players 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.”
And there you are. Go get creative. Make a new gun, or something.