Not to boast, but I've been happy with the word-of-mouth advertising I get at Theme Hybrid. I buy absolutely no ads, have no affiliate program, and don't really promote the site that much myself. I've been extremely happy with the support of the Theme Hybrid community getting the word out and letting folks know about their experiences.

If I had to choose another theme site that has great word-of-mouth marketing, probably the best, it's StudioPress. But, WooThemes definitely has the best marketing campaign going on.