GFXedit.com: WordPress themes for free download. Themes use CSS and are xHTML compiant.
Graphical Theme Viewer: Collection of WordPress themes with a graphical and textual preview available for each theme. Sorting and search feature as well.
Kickass Web Design: WordPress themes for download. Individual theme author. Custom work available for a fee.
PlainTxt.org: Designs free, downloadable WordPress templates that offer a unique, minimalist approach to blogging. Themes are WordPress 2.0+ compatible.
WordPress Graphics Company: Templates suitable for corporate and professional blogs.
Wordpress Theme Browser: Collection of WordPress 1.5 themes released under GPL compatible licenses as part of the Alex King's theme competition.
WP.BOARD: Unique WordPress themes released on a periodic basis. Annual membership payment required to download themes.
WPThemes.Info: Customizable WordPress themes, some for sale and some available for free download.