BuddyPress - WordPress social networking in a box.
SocialEngine - Socialengine PHP: Elevating Your Online Community Experience.
PeepSo - PeepSo is a FREE WordPress plugin which allows you to easily create gorgeous user profiles and spectacular communities within your WordPress site.
phpFox - phpFox is a powerful social network platform for niche communities
Ultimate Member - The easiest way to create powerful online communities and beautiful user profiles with WordPress.
bbPress - bbPress is forum software with a twist from the creators of WordPress.