Pligg CMS Pligg is an open source CMS (Content Management System) that you can download and use for free. Pligg CMS provides social networking software that encourages visitors to register on your website so that they can submit content and connect with other users.
Elgg Elgg is free and open source software. It provides the components you need to build a fully-featured social environment.
insoshi insoshi is a social networking platform in Ruby on Rails. You can use Insoshi to make custom social networks.
XOOPS CMS XOOPS is a web application platform written in PHP for the MySQL database. Its object orientation makes it an ideal tool for developing small or large community websites, intra company and corporate portals, weblogs and much more.
Community Engine CommunityEngine is a free, open-source social network plugin for Ruby on Rails applications. Drop it into your new or existing application, and you?ll instantly have all the features of a basic community site.