Joomla!
From Organic Design wiki
Joomla is a free and open source content management system (CMS) for publishing content on the Web and intranets. It comprises a model–view–controller (MVC) Web application framework that can also be used independently.
Joomla is written in Object-oriented PHP, and uses software design patterns, stores data in a MySQL, and includes features such as page caching, RSS feeds, printable versions of pages, news flashes, blogs, polls, search, and support for language internationalisation (i18n).
Within its first year of release, Joomla was downloaded 2.5 million times. Over 6,000 free and commercial plug-ins are available.