This is the start of a future directory that will list the top or most popular themes in various categories. The listings here are not limited to the Drupal world themes, in order to encourage the Drupal community to seek best practices everywhere.
Be sure to add links to where those themes can be found!
All themes
Drupal themes
Garland (core)
Marinelli
Acquia Marina
RootCandy [admin theme]
Sky
Pixture Reloaded
Interactive Media
Fluidgrid - a flexible fluid Grid based theme
Other resources
Top Drupal themes
Best Themes
A review of Drupal 6 starter themes
Drupal 6.x starter theme comparison