DiamondWiki's Faceted Navigation:

"Faceted navigation lets people browse a website by using FacetedClassification to automatically generate relevant hyperlinks. If you want to see an example of faceted navigation in action, go to the BrowseFacets page and start clicking, paying attention to the categories on the left-hand side. Notice how pages can have both an "Author" and a "Subject", and you can navigate by either one. This may seem obvious to you, but the point is that pages are not restricted to a single position in one hierarchy -- this is what faceted classification is all about. It's nothing earth-shattering.

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."