Yahoo! has a Search Engine Optimization (SEO) Resources section, where they list out sites included in the directory in order of "popularity." Here is it...
What makes one site more popular than the next? Let's look at another example close to home. The Internet Consultants New York section has my business ranking number one, above Agency and Razorfish.
So why isn't Evolution.com number one?
Let's look at the SEO category:
Search Engine Watch has 213,437 external links via Site Explorer. WebmasterWorld has 275,587 external links via Site Explorer. SEOBook has 283,855 external links via Site Explorer. Search Engine Roundtable has 239,111 external links via Site Explorer.
All about the same.
How does Yahoo! define popularity?
By default, Directory site listings are presented sorted by popularity and relevance. Sites that are most popular with users or the most relevant to the category appear at the top of the site listings. The order of web sites or web documents is based upon Yahoo! Search Technology. If the category is large, the listings will display over multiple pages with pagination displayed at bottom of the page.
Sagerock at the WebmasterWorld suggest it is about click popularity. But I suspect it is a bit more detailed based on Yahoo! saying, "order of web sites or web documents is based upon Yahoo! Search Technology."
They can easily apply a popularity algorithm to the Directory and order pages by that. Now if we knew exactly how the popularity algorithm worked, then you know...
Forum discussion at WebmasterWorld.