Fallout 3 was initially under development by Black Isle Studios, a studio owned by Interplay Entertainment, under the working title Van Buren. Van Buren was set to carry on the Fallout series, following Fallout 2, prior to being cancelled. Interplay Entertainment went bankrupt and closed down Black Isle Studios before the game could be completed, and the license to develop Fallout 3 was sold for a $1,175,000 minimum guaranteed advance against royalties to Bethesda Softworks, a studio primarily known as the developer of the The Elder Scrolls series. Bethesda’s Fallout 3 however, was developed from scratch, using neither Van Buren code, nor any other materials created by Black Isle Studios. Fallout 3 was to be centered around Utah and Colorado, but it also would have included areas of neighboring states. The player would be able to visit places such as Hoover Dam, Denver, Mesa Verde, and the Grand Canyon. [Info from Wikipedia]
In May 2007, a playable technology demo of the cancelled project was released to the public. You can read more about and download the beta at No Mutant Allowed website. Various unused maps, weapons and models can still be found in this Alpha-demo thanks to some codes / hacks. Also, the full Fallout 3: Van Buren Design Doc was leaked online, you can find it in here!
In the videos below you can even see Chris Avellone talking about the project at the Framework 2009 convention.
Thanks to Robert Seddon and Megalol for the contributions!