SocialEngine - Socialengine PHP: Elevating Your Online Community Experience.
Higher Logic - Private, secure online communities that stimulate conversation and ignite engagement.
mooSocial - mooSocial is the best social network script to create a niche community or social site.
Tribe.so - Tribe is a white-label platform, empowering the new generation of online communities.
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.
Discourse - Discourse is an open source discussion platform built for the next decade of the Internet.