Fallout 3 is a action RPG developed by Bethesda Game Studios in 2008, for the Xbox 360, Playstation 3 and PC. As we can read on Wikipedia, the F3 project was initially under development by Black Isle Studios, a studio owned by Interplay Entertainment, under the working title Fallout 3: Van Buren.
When Interplay went bankrupt and closed down Black Isle Studios before the game could be completed, 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 for 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.
In the released game there are still many unused items, weapons and perks, while in some early videos we can notice various beta locations and characters. You can find the full list of unused items at Fallout Wikia!
The following YouTube videos show cut content and development content from when fallout 3 and its DLC was in development. Very interesting stuff; almost all of the cut content can be accessed on the PC version and the beta tesla cannon can be found in the broken steel DLC and used on all ports of fallout 3 too.
Thanks to DCodes7 for the contribution!