Drupal 6 theme – LightFantastic – was developed for the Google Summer of Code 2007. It has a flexible structure and can be customized to have single, two- or three-column layout, fixed- or fluid width either. As it is a beta release, you can check out this info for future references.

Free Drupal 6 theme – I feel dirty – was initially designed for WordPress by Studio ST. It has a grunge look, single right sidebar, discrete header/footer regions, fixed layout, nicely styled comments and comment forms. All that in a whole make this drupal template look prominent.