Noosfero!

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)

Social networking in a box. Build a social network for your company, school, sports team or niche community all based on the power and flexibility of WordPress. Install BuddyPress directly through your WordPress dashboard plugins page, or download it manually from the WordPress.org plugins repository.

ImpressPages is a PHP framework with admin panel. A new MVC engine is under the hood so you can customize ImpressPages in no time. Write your own plugins and themes to suit your and customers’ needs best. Both website layout and administration interface are user friendly and simply gorgeous.

Symphony is an XSLT-powered open source content management system

SPIP is a publishing system for the Internet in which great importance is attached to collaborative working, to multilingual environments, and to simplicity of use for web authors. It is free software, distributed under the GNU/GPL licence. This means that it can be used for any Internet site, whether personal or institutional, non-profit or commercial.

The award-winning Open Source Community software for Joomla. JomSocial is a powerful social networking software, loaded with plenty of social network features to empower your community such as entrepreneur circles, gamers clubs, hobbyist communities, artist fan pages and a lot more!

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

BlazeBlogger is a simple to use but capable CMS for the command line. Being written in Perl as a cross-platform application, and producing static content without the need of database servers or server-side scripting, it is literally a CMS without boundaries suitable for a wide variety of web presentations, from a personal weblog to a project page, or even a company website.

phpSQLiteCMS is a simple and lightweight open source web content management system (CMS) based on PHP and SQLite. It's licensed under the GNU General Public License.phpSQLiteCMS is good if you want to quickly set up a small website without needing to spend a lot of time studying, installing and configuring (as SQLite is file-based, it just runs "out of the box"). But note that it is no large scale content management system - it's a tiny and simple one!

Social Network For Joomla to grow your communities with the least time.

Wagtail is a content management system built on Django. It's focused on user experience, and offers precise control for designers and developers.

Google App Engine Social network.