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Elgg is an open source social networking framework. It provides the necessary functionality that allows you to run your own social networking site, whether publicly (like Facebook) or internally on a networked intranet (like Microsoft Sharepoint). To run Elgg, you need to have Apache web server, PHP5.2+ and MySQL 5+.

Elgg comes with advanced user management and administration, social networking, cross-site tagging, powerful access control lists, internationalization support, multiple view support (eg cell phones, iPhone), an advanced templating engine, a widget framework and more. You can also use plugins to add more features like: Blogs, Bookmarks, File repository, Message board, Private messaging, Pages, Activity, Microblogging, etc.

Features

• User, object, file and site management
• Social graph functionality (relationships between users and other users, objects and sites)
• Easy internationalization support
• System-wide, tag-based searching across all content and users
• Fine-grained access controls
• Multiple views, allowing for mobile applications and embeddable widgets as well as the traditional web browser view
• Plugin APIs
• Powerful theming through the plugin API
• RSS and FOAF for content syndication
• OpenID and LDAP for user authentication
• OpenSocial
• An extensible RESTful API, with results in JSON, serialised PHP or XML
• AJAX through jQuery and user-definable callbacks
• Easy extension for use with caching systems such as memcached, for increased system performance (experimental)
• Use of multiple database connections for scalability (still in development)

Developed by Curverider; Elgg is available for download under under the GNU Public License.  You can find further information, documentation & download on Elgg Website.