Vivisimo is an engine that "auto-categorise" the search results from several other search engines. I don't know how but I don't care too much either. What I like about it is that it shows several categories of the search results; as opposed to Google where all returning items are listed one after another. When I search for Peter Bengtsson it then creates several categories such as "Zope", "Family", "Peterbe.com", etc. Some don't make sense but suppose a friend searches for "Peter Bengtsson" but is only interested my person stuff, or a Zope developer is interested in my stuff on Zope then this clustered search engine beats Google.
I also like the "preview" option that you see for each search result.
But maybe they have changed somethings to make it work differently.