Ning is very cool.
Ning is a free online service (or, as we like to call it, a Playground) for building and using social applications.
Social apps are web applications that enable anyone to match, transact, and communicate with other people.
Basically, you can clone any application on Ning in minutes, and modify it for your own city, niche, or whatever. I think this is part of a larger trend – moving towards niches for everything. It's one way to deal with information overload. Someday, the only markets we may have will be niche markets.