Drupal is one of the most advanced open source CMS solutions available today. Originating in 2001, it has release 5 successful versions and is moving quickly towards its sixth.
One of Drupal's characteristic features is is modular design. By virtue of its object oriented structure, security and feature improvements can be implemented as modules running on top of the core installation without the need to modify the core code. This allows for greater ease when updating the software, and avoids legacy nightmares.
Its core installation provides an extensive set of features, ranging from administrative tools such as user management and caching to content creation tools like collaborative writing and poll generation. By adding other custom modules, other features such as e-commerce packages and photo galleries can be easily added. Drupal's community contributes many such modules which are available for free download.
If you are looking for a CMS which provides a framework on which to write customize applications, Drupal is a great way to go. If you are simply trying to run a blog and update static pages, you may be better off with WordPress.
Drupal is written in PHP and runs on Apache or IIS.
Homepage: http://drupal.org
Download: http://drupal.org/project