Om points to what sounds like an incredible leap forward in the web 2.0 space...a meta-application framework for developing social network applications.
From Ning's homepage: "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."
This will supposedly give one the ability to build your own "take" on flickr, for instance.
Just looking through the Ning site at what people are building with the framework is interesting...look for the "Pivot" section.
This meta-app, if it works as advertised, seems like it has the potential to spawn an even bigger wave of niche development. I imagine this will cause a ripple at the Web 2.0 conference today. It will be fun to track the feedback that rolls in...especially since Andreesen is one of the marquee names involved.