Drupal 6 Theme Test-driver
Reading through the Weekly Drupal feed, I found a linked site that is pretty rad.
Being primarily a developer, I am always interested in exploring the way the designer's side of Drupal. Besides helping me understand Drupal more fully and write more easily extensible code, its fun/cool to make sites look fun/cool.
Check out Theme Garden. It allows you to window shop different themes and their layouts, while the content stays the same. Simply click a listed theme to view it. Pretty rad.