It has many names; Faceted Search, Refinement UI, Search within Search and it’s a feature that’s frequently requested.  Both Search Server 2008 Express and SharePoint Server 2007 Search can display faceted results with just a little customization work using the samples available on CodePlex here -
http://www.codeplex.com/facetedsearch

Facets are implemented and administered as managed properties, so HTML pages, Office documents, SharePoint lists, database tables, web services - in fact, any content source indexed by MOSS can be made available as a facet.

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Leonid Lyublinski
MCS Consultant