We'll update this list from time to time. Here are some places to look when searching for WordPress themes:
WordPress.org Theme Directory - all themes listed here are free. For many of these themes, the WordPress default "preview" doesn't do them justice. If you know how to hunt, you can often find some nice themes here.
StudioPress is the home of the Genesis Theme framework. Child Themes built on top of Genesis are generally coded according to WordPress best practices and make a great foundation for a well-built WordPress site.
Themeforest WordPress Themes - Your mileage will vary with Themeforest WordPress Themes. While all of them look great, sometimes they can be a pain to work with. Before purchasing a Themeforest WordPress Theme consult with us or another WordPress expert.