"An essential part of FacetedNavigation is that the interface lets you view items that are in more than one category. In other words you can intersect two sets of items. So for example, you can view "items about diamond wiki that are authored by kim burchett", instead of being restricted to viewing "items about diamond wiki" or "items authored by kim burchett". Most hierarchical categorization systems only let you view one hierarchy at a time." -- http://www.kimbly.com/cgi-bin/diamond/FacetedNavigation
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PeriPeri seems to have something like this.
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CategoryWikiNavigation