Faceted Search

Leonid Lyublinski from Microsoft has published a powerful extension to MOSS search that allows you to drill down into the search results using indexed metadata: http://www.codeplex.com/FacetedSearch

Here is the MSDN blog post: http://blogs.msdn.com/sharepoint/archive/2008/01/30/open-source-faceted-search-for-moss-2007-and-microsoft-search-server-2008-part-1-of-2.aspx and WSSDemo has implemented it on their site: http://www.wssdemo.com/Search/Pages/results.aspx?k=search