One of the first tips on creating themes referenced devcowsoftware.com,
however they have since added a theme site which implements the GraffitiExtras Theme Explorer widget/plugin. This allows you to preview the theme before you download it. They are also accepting theme designs to add to their growing list of free themes.
Their new Graffiti Theme site is located at http://themes.devcowsoftware.com/. Check it out, and if you have developed a theme and would like to share it, email [email protected]