Drupal is a PHP CMS that supports MySQL and PostgreSQL
That didn't seem to be clear to some of the folks at the recent Barcamp-Amsterdam, so for the record, there it is.
Drupal is a fantastic too
People with low technical skills can use it to make very dynamic, community-oriented websites with multi-user blogging and forums. There are over 100 contributed modules meaning it can do *just about anything*. From a developer's point of view, it is wonderful to have so many of the common web application challenges abstracted into a reusable API that can be used either to extend Drupal, or as a library in other web applications.
-- Database managed authentication and sessions, meaning it is a great framework for distributed/mirrored/clustered webapps.
-- Page caching for high performance websites.
-- Database abstraction layer so that code is reusable with many RDBMSs and some common types of security holes (SQL injection) are eliminated at the framework level.
-- Fully accessible API via webservices (REST/XML-RPC).
-- HTML forms API
-- Hierarchical taxonomy API for managing categories, keywords and relations of all natures
-- Granular user/role permissions system
-- Permissions extensible to the micro-content level.
Best of all, the community rocks.