BuddyPress - WordPress social networking in a box.
SocialEngine - Socialengine PHP: Elevating Your Online Community Experience.
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
MemberPress - Easy to use WordPress Membership Plugin. Powerful Membership Software seamlessly integrates with WordPress, Stripe, PayPal and Authorize.net.
HumHub - HumHub is a free social network software and framework built to give you the tools to make teamwork easy and successful.