Introducing New Vegas: Multiplayer, a mod that will let players venture out into the Mojave wasteland with buddies (or enemies). While the modification of the 2010 game is still in its early days, what developers are showing off so far looks promising. Here, for example, you can watch a few different players all load into a single area, as well as some combat:
You'll note that nobody is using VATs. For now, that's been turned off in the mod, which makes sense -- it sounds difficult to make a mechanic like that work against other players. Pipboys may still be used, but instead of pausing the game, it will turn you invincible and immobilise you, so that other players can't harm you. Other details remain in full, however -- note, for example, how players can pickpocket each other.
It gets better than that...or worse, depending on your definition. Over on Reddit, Mr. Mouse, one of the developers, notes that the game is set up so that players can affect each other's questlines:
The way it works currently is that quests are clientside, they still take place in the world but one player in a location will synchronise entities/NPCs to other players around them. This means that if you entered an area where a quest NPC should be, and someone has previously killed them in the cell (whilst someone is currently in it with you), then you will fail the quest on entry.
It sounds like the makings for some DayZ type hijinks, only in Fallout -- which sounds exciting to me.
The entire thing is still running off Gamebryo, but Mr. Mouse notes that the engine has been exploited to "produce a working multiplayer environment". It's rough at the moment, but hopefully they can polish it up some more.
Right now, New Vegas: Multiplayer is looking for volunteers for a gameplay test in a couple of weeks, and you can sign up for that here.