Drupal

Drupal is an established, open-source Content Management System, with a huge library of modules. First released in 2001, the official Drupal web site boasts over 350,000 members, with hundreds of maintainers and contributors. Drupal's stability and modular structure allows it to be used for anything from large corporate sites such as AOL's corporate portal, to news sites such as Popular Science, to numerous personal web sites.

Drupal is licensed under the GPL. It is implemented in PHP, and uses either MySQL or PostgreSQL. It can run on either Apache or Microsoft IIS.