Virtual worlds. The "metaverse" of Snow Crash. "Snow Crash", the TV series, has been green-lighted by Amazon. That may drum up more interest. Second Life comes close. They've had a working system for over a decade and have with adequate solutions to most of the problems. But it's hard to use, and not that many people are interested.
I never got the value proposition of Second Life. It wasn't game-y enough to be a game and, for me at least, it didn't really add much over more "traditional" collaboration/communication/socialising methods like text and video chat, social networks, twitter etc.