Some real nice collections:
http://drupal.org/project/Themes
http://drupal2u.com/
http://www.tripwiremagazine.com/2009/05/200-massive-free-drupal-theme-compilation.html
http://www.tripwiremagazine.com/2009/08/225-ultimate-drupal-theme-collection.html
http://www.freedrupalthemes.net/
http://www.freedrupaltheme.com/
http://www.drupalthemesdownload.com/
