By the time you're reading this, Fallout: London is available to download right now. The mod we've all been waiting for was meant to release this April, but Bethesda being Bethesda released an update that essentially sent the creators back to the drawing board.
After years of development, the mod is finally here. It'll have its own story and world to explore, fully voiced NPCs, new enemies to fight, new weapons, and a functioning tube system. It's potentially the most ambitious Fallout mod of all-time, and it's out now.
The mod is available to download from GOG here. It's worth noting that if you have the game on Steam, you'll have to reverse it to a pre-patch state, before the next-gen update came out before you can play, while the GOG version of Fallout 4 runs fine with it.