Rubik is a clean admin theme. It features a set of icons for admin pages provided by Drupal core and aggressive styling to reduce visual noise wherever possible.
It is one of the most popular admin theme replacements for Drupal and is reported to be installed on over 15,000 sites.
Tao is a base theme for Drupal that is all about going with the flow. It is not
meant for general-purpose use, but instead takes care of several tasks so that
sub-themes can get on with their job:
Designer Relief is a Wordpress theme that I converted to a Drupal 6 theme. It's great for use as a blog theme, and has some customizations done to the page.tpl and node.tpl files to allow for the date and comments to be displayed differently than they normally are in themes. It's very close to the original Wordpress theme, but has a few minor adjustments I made to Drupify it a bit. I would say that it is 95% completely converted. I did add a new region to the theme, which was perfect for the Primary Links/Main Menu. It is located across the top of the theme in the wood area.
This module provides a field type for referencing terms with a level to an entity, e.g. someone can add the term "Java" with the level "Expert" to a node.