What other big time fan made mods are there for games? (Similar to Fallout London)

[Kane99]

With the recent release of Fallout: London, the big mod released for Fallout 4 that adds a completely new area (London) and a whole new story and cast of characters. And this is all made by fans of the game. But, are there other fan projects similar to this one, where someone has made a mod that is like a completely new game? I like when fans and gamers keep an old game alive, whether that means to add new story missions, quests, characters, locations, etc. Are there any fan communities that have put out massive mods like Fallout: London, but for other games? If so, what games are being kept alive like this by fans? 

[killamch89]

Three that come to mind are Black Mesa for Half-Life, NeoTokyo for Half-Life 2 and Dayz for Arma II which are very huge mods that changed large aspects of the original game to create a far different experience. There's also Skywind and Skyoblivion coming out for Skyrim in the near future.