Garden of Eden Creation Kit

The Garden of Eden Creation Kit (GECK):

The GECK is used to edit and create game content for Fallout 3 and Fallout: New Vegas. The data is stored in data files that are read by the game. The GECK allows you to create your own cells and worldspaces (towns, dungeons, etc.) then populate them with your own characters, creatures, items, and stories.

GECK is included with the GOG releases of both games, Steam users can find the program by searching it in the Tools section of their library.

GECK can easily be found on Steam by searching "New Vegas"

Official download links are below for users of the Epic Games Store version of Fallout 3 and Fallout: New Vegas.

Download the Garden of Eden Creation Kit here.
Download the 1.5 Update here.
Download the Garden of Eden Creation Kit for Fallout: New Vegas here

For set up and configuration of the GECK, see here