Since you are here, you probably know about Drupal. It is content management system, released as open source. Drupal powers large number of sites on the Internet, like
Drupal supports themes, enabling control of look and feel to site administrator and even users. For some time drupal.org run the Theme Garden as a kind of Drupal themes beauty exhibition. Unfortunately, with release of Drupal version 5.0, this site was closed down.