BuddyPress: Open Source Social Networking
BuddyPress is a collection of plugins for WordPress Multi-User edition that provides social profiles and networking features. Users, groups, blogs, activity, friends, and all the normal features are included, with a very clean user interface.
A BuddyPress test site is online to let interested parties see how it works in action. (It even includes a group interested in open source social networking, including competitors like Elgg and others, which is cool.)
Just as WordPress itself brought open source blogging to the mainstream, it looks like BuddyPress could do the same for distributed, open source social networking. This will be a fascinating, innovative, and in my mind, long-awaited development.