Well, since themes.org has lapsed lately, there is a new site out of the UK that is becoming a theme center. http://www.sunshineinabag.co.uk has gtk, gtk2, sawfish, meta themes, as well as nautilus themes and emblems. A very good site, still being filled up with new themes and in need of some minor UI features (new themes at the top of the page for example :) but a great resource nonetheless.