The people at WP Candy have put together a great tool for helping people find WordPress themes. ThemeFinder lets you do a quick visual search by color, layout, and price (free or paid). You simply select filters from the top of the screen and the tool highlights thumbnails that fit your criteria. Click any link to view a larger version of the them at the author’s site.
It’s not an inclusive list by any stretch, but I think they did a good job focusing on quality. They include work from several theme shops that I didn’t know about. It’s a fun tool, and it’s definitely worth a look.