Joomla! is a fork of Mambo, a popular open source CMS. It is also open source and is published under the GPL license. A new version of Mambo based on the CakePHP framework is rumored to be in development.
Joomla's PHP code is structured with an emphasis on modularity to make it as flexible as possible. It requires MySQL, which is great for most shared LAMP (Linux, Apache, MySQL, PHP) hosting environments, but offers no alternative at this time.
Joomla! has a large community with thousands of websites providing free templates and plugins. Like Drupal, if you are looking for a good basic to write a highly customized CMS, Joomla is worth a look. If you want to run a simple site, WordPress may be more suited to your purpose.
Homepage: http://joomla.org