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.
Useful Drupal module to assign separate themes for different roles (including anonymous) in your system.
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.