Joomla! is a primary example of open source being used to build and maintain a tremendously powerful system for managing large amounts of content and providing a solid framework for functions like blogging, social networking, ecommerce, directories, and more. With over 4,000 third-party applications, more than 10,000 active developers, and well over 100,000 practicioners, Joomla! is a sure bet for the success of almost any website.
Joomla! is the rebirth of the wildly popular Mambo content management system. A few years back, Mambo was forked into two separate projects: Mambo (a commercial enterprise), and Joomla!, and entirely open-source community owned platform for content management.