Bethesda recently released an editor for their latest release: Fallout 3. Cleverly named after one of the components in the game, the G.E.C.K (Garden of Eden Creation Kit) allows PC players to manipulate most aspects of the game, including dialogue, landscapes, terrains, textures, characters and more.
Similar to some of Bethesda's other games, the game has now been opened up (to a certain degree) for modification, and already there have been small modifications released. Fans of the game can expect to see a plethora of new creatures, clothes, terrains and textures, as there was for Oblivion (which now has thousands of user made modifications).
Bethesda has also set up a tutorial and help site for the content editor, which has an already large collection of guides and reference pages (over 1000 at the time of writing) to help fans get started. The entire kit is available free of charge and in a tiny 8MB package too.
The game already has more than enough content for the average player, but this kit is sure to extend the lifetime of the game, for PC gamers only, at the moment, but extra (official) content may be available for console editions in the future.