Teoma is another great example of clustering search engine. They developed a different marker for establishing the rank of a page. Instead of the PageRank developed by Brin and Page, they are using a Subject Specific Popularity:
Instead of ranking results based upon the sites with the most links leading to them, Teoma analyzes the Web as it is organically organized-in naturally-occurring communities that are about or related to the same subject-to determine which sites are most relevant. Teoma is the only search technology that can locate communities on the Web within their specific subject areas, as they actually exist. And this allows us to finely tune our search process, providing more precise results.