Genesis is an standards compliant, accessible and semantically rich starter theme for Drupal 6. Genesis is easy to theme yet extremely flexible, giving you endless theming possibilities. Genesis makes building a standards compliant accessible theme easier than starting from scratch.
A Drupal admin theme created by Jeremy Caldwell. A mix of Google and Wordpress ideas and colors in an easy to use and configure layout.
Useful Drupal module to assign separate themes for different roles (including anonymous) in your system.
RootCandy is an experimental admin theme for drupal 6
The taxonomy_theme module allows you to change the theme of a given node based on the taxonomy term, vocabulary or nodetype of that node. You can also theme your forums and map themes to Drupal paths or path aliases directly.
Wabi theme is a original recolorable theme for Drupal 5.x and 6.x