Bethesda has released the official mod tools for Fallout 3. Cleverly called G.E.C.K., this Garden of Eden Creation Kit will allow PC players to create all manner of ridiculous modifications.
The company has also built a wiki site devoted to tutorials to help people get started. If you're interested in downloading the tools and getting started, this text I am typing right here is the link to said tools.
So what do you want to see people do with Fallout 3? Maybe they could import Oblivion's environments and quests into it and actually let you play Oblivion With Guns. That'd be cool. Who needs Daedric artifacts when you've got portable frickin' nukes?