Top 50 Free Wordpress Theme (Excellent article, has screen shots of each theme, with a link to a demo, and a download link)

100 Excellent Free Wordpress Themes (Great article on 100 Free Wordpress themes, shows screen shots of some)

20 More Free First-Class Wordpress Themes

83 Beautiful Wordpress Themes

10 Fresh and Clean Themes

10 Unusual & Original Wordpress Themes

21 Fresh, Usable and Elegant Themes

Top Wordpress Themes (Great site, split up into interesting categories such as 2-column, 3-column, widget ready, etc)

My Top Thirty Wordpress Themes

Best 3-Column Wordpress Themes

The Best Minimalist Wordpress Themes

Top Ten Best Free Wordpress Themes - Part 1 :: Part 2 :: Part 3

10+ Free Magazine Style WordPress Themes to Choose From

Top 50 Rated Wordpress Themes

Best Wordpress Themes on Squidoo

Top 20 WordPress Themes of 2007

10 Best Web 2.0 Wordpress Themes

This last bunch is from Hackwordpress.com

1. Premium WordPress Themes Gallery
2. 2-Column WordPress Theme Gallery
3. 3-Column WordPress Theme Gallery
4. 4-Column WordPress Theme Gallery
5. K2 WordPress Theme Mods and Styles Gallery
6. Google AdSense Optimized WordPress Theme Gallery
7. Search Engine Optimized (SEO) WordPress Theme Gallery
8. Magazine Style WordPress Theme Gallery
9. 125×125 Button-Ready WordPress Themes Gallery