BRAINFART

Free 3d-party CMS.
Official site: http://drupal.org.
Advantages: templates control; system architecture; API; taxonomy; community.
Disadvantages: difficulty of working without preparation.

If you wish to completely control appearance and functionality of your site (level of control depends only on your desire), and thus not using dirty methods (like hacks of initial code of the core and modules) and to work only through API and extremely flexible template system – Drupal is for you...