SocialEngine - Socialengine PHP: Elevating Your Online Community Experience.
BuddyPress - WordPress social networking in a box.
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.
MemberPress - Easy to use WordPress Membership Plugin. Powerful Membership Software seamlessly integrates with WordPress, Stripe, PayPal and Authorize.net.
mooSocial - mooSocial is the best social network script to create a niche community or social site.