Drupal

From Wikipedia:
Drupal (pronounced /ˈdruːpəl/) is a free and open source modular framework and content management system (CMS) written in the programming language PHP. Drupal, like many modern CMSs, allows the system administrator to create and organize content, customize the presentation, automate administrative tasks, and manage site visitors and contributors. Although there is a sophisticated programming interface, most tasks can be accomplished with little or no programming. Drupal is sometimes described as a "web application framework," as its capabilities extend from content management to enabling a wide range of services and transactions.