BuddyPress 1.0 (Transforms your WordPress blog into a social network platform.)

BuddyPress is a suite of WordPress plugins and themes, each adding a distinct new feature to let members socially interact, use all of the plugins to create a complete social network, or specific plugins to add desirable features to your existing blog.

http://buddypress.org/

“Note that Buddypress requires WordPress Mu.”

http://mu.wordpress.org/