Oxwall!

Diaspora is a federated social networking service, designed to be a more privacy-aware alternative to Facebook . Share what you want, with whom you want.

Friendica is a distributed social network application which runs on commodity hosting platforms - PHP/MySQL/Apache. (If you can install Drupal or Wordpress you can probably install Friendica)

Elgg is a leading open source social networking engine which can be used to power your social network.

Ning is the social platform for the worlds interests and passions online. Millions of people every day are coming together across Ning to explore and express their interests, discover new passions, and meet new people around shared pursuits.

Powerful Social Networking SoftwareWhatever you are passionate about, build your community with Jamroom - the most powerful and flexible social networking software.

Sharetronix is a Secure Social Network for Your Company. Collaborate with Your Coworkers, Clients, and Partners.