Fallout: The Frontier is now available, offering an ambitious modification for Fallout: New Vegas. This requires ownership of the original game on Steam as well as all four of its official DLC packs. You can download the mod through Nexus Mods and it is also coming soon to Steam as well, but they say the Steam release is delayed "for a very short time" while they work out some build issues. Here's a Release Trailer, and here's more on this project that they call a "game sized modification":

In the cold, snowy streets of Portland Oregon, the New California Exiles and Caesar’s Legion fight for the domination of a city rich in resources and military hardware, but what lies in wait under the snowy ruins...

The Frontier is a new game sized modification for Fallout: New Vegas. Created by a passionate group of dozens of community members over a seven year period, The Frontier is the largest modification ever released for Fallout: New Vegas, and aims to be a unique experience for players both old and new.

Featuring new gameplay mechanics such as high quality drivable vehicles, and incredible scripting and visual effects that are sure to impress even the oldest fans of the series. All of this wrapped up with high quality voice acting, dozens of side quests and hundreds of exciting new characters!