BuddyPress.org

In SocialGo’s defense, there’s nothing wrong with asking money for a service/product. SocialGo and Ning are good solutions for a certain class of users. The difference is “software as a service” vs self-hosted.

Another option is SocialEngine, which is self-hosted but not free and probably a bit more business-ready than Buddypress because of it – better member mgmt, anti-spam, photo + event plugins, etc.