Joomla!
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Joomla! is a free, open source content publishing system designed for quickly creating highly interactive multi-language Web sites, online communities, media portals, blogs and ecommerce applications. Joomla! provides an easy-to-use graphical user interface that simplifies the management and publishing of large volumes of content including HTML, documents, and rich media. Joomla! is used by organizations of all sizes for Public Websites, Intranets and Extranets and is supported by a community of thousands of users. With fully documented library of developer resources, Joomla! allows the customization of every aspect of a Website including presentation, layout, administration and rapid integration with third-party applications.
Further information at: http://www.joomla.org