Vivisimo is said to be a new search engine that could. They re-launched with a new name at a site called "Clusty.com", which I gave a spin just now. It's interesting. The search result comes up with "clustered" related categories on a side panel, so you can go further into the search if you wish. This small convenience of having a "related categories panel" on the same page is something Google and other search services might imitate in the future.