StÈfan has a blog entry on Dogpile Search Comparitor tool which produces a venn diagram of the results from different search engines. The interface, however, has a fatal flaw. You can’t link to the resulting sites that are common to more than one search engine (and presumably are the ones you want). The little pills in the middle – which stand for the shared results link to a Dogpile listing, not to the resulting site itself.