BuddyPress is a social networking application built on top of WordPress Multi-User (MU) as a plugin. WordPress MU is the the software behind WordPress.com. It allows a user to host a network of blogs on a site. BuddyPress adds a large variety of new social interaction features to WordPress. The features can be added all at once or as individual components:
A WordPress blog
Individual profiles for members
Private Message (PM) functionality
Friend Networks
Groups, which include a blog, photos and other user generated content
“The Wire” (like the wall on Facebook)
Activity/Action streams of public actions on the site
Forums using bbPress
A fully featured demo is available at the BuddyPress website.
Codex has some developer documentation
