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.
JomSocial - Open Source Social Networking Software. Award winning open source community software by JomSocial. This social networking software is widely used by many Joomla users.
Ultimate Member - The easiest way to create powerful online communities and beautiful user profiles with WordPress.
phpFox - phpFox is a powerful social network platform for niche communities