BuddyPress

This looks very exciting.

Thanks to Nick for the tip off.

Edit: More via TechCrunch:

It is easy to dismiss this as completely unnecessary given the abundance of social networks already out there, as well as application development platforms like OpenSocial. But an open-source social network does present some intriguing possibilities. New apps and features could be added simply by creating new plugins. And there would be no lock-in to any proprietary code or development environment.