Invision Community - Invision Community is a flexible, integrated set of apps for building communities on the web.
SocialEngine - Socialengine PHP: Elevating Your Online Community Experience.
XenForo - Intuitive. Social. Engaging. Fast. XenForo brings a fresh outlook to forum software.
BuddyPress - WordPress social networking in a box.
Discourse - Discourse is an open source discussion platform built for the next decade of the Internet.
phpFox - phpFox is a powerful social network platform for niche communities