Drupal is yet another open source CMS (Content Management System) for PHP and MySQL (It can also work with other databases like Postgress). Drupal is one of the most mature, PHP based, CMSs out there. We've been working with it for various clients since 2003 (it was originally released in 2001)! Currently released at version 6 (which is quite the wise, old age in "internet time"), the Drupal Core is rock-solid with a very large and loyal community supporting and contributing a wealth of plugin component modules and solutions for it.
Drupal's strength lies it's ability to organize and manage a large variety of content and user-types within a single site and system (it also as "true" multi-site features you can run many sites from one Drupal install). For very large organizations with many different types of users and editors, robust portal sites and online catalog/commerce sites, Drupal is a capable CMS and worth taking a close look at.