Here are some of the popular and not so popular Wordpress themes directories with short reviews.

http://wordpress.org/extend/themes/ : The official Wordpress.org themes section. Features search by number of columns, primary colour of the theme, theme topic, fixed or flexible width and some of the features like custom-colours, theme options, translation ready, buddypress and so on. Features most of the themes but misses out on some of the great free themes offered by commercial Wordpress theme developers. No information about the version of Wordpress the theme is developed for.