Drupal
Drupal is an open source Content Management System, developed in PHP and published under GNU/GPL, with a good and clean design, powerful and flexible.
This is my CMS of choice, after testing a number of them and comparing usability, performance and code. I got involved in the development contributing some modules and I have some related projects in progress based on Drupal: a fully customized and preconfigured version for Intranets and another one to build corporate sites.
More info about Drupal on www.drupal.org.
This is my Drupal.org user.
On Drupal Hispano (Spanish Drupal community), I am a site maintainer.
