There are Fallout mods, and then there's Fallout: New California.
Fallout: New California, aka project Brazil, is a mod over five years in the making. It totally converts Obsidian's Fallout: New Vegas and launches for free on PC on 23rd October - potentially around the same time Bethesda's recently-announced Fallout 76 comes out.
Fallout: New California tells a prequel to the wonderful Fallout: New Vegas. It's set in the year 2260 and begins in Vault 18, which is set in post-apocalyptic Los Angeles. As you'd expect, you make your way to the surface where you find a number of factions fighting for control over the California wasteland.
