Joomla is an award-winning content management system (CMS), which enables you to build Web sites and powerful online applications. Many aspects, including its ease-of-use and extensibility, have made Joomla the most popular Web site software available. Best of all, Joomla is an open source solution that is freely available to everyone.

joomla
http://www.joomla.org/
http://www.joomla.org/download.html
http://www.joomla.org/technical-requirements.html