User Interface Implementations of Faceted Browsing
Over the last couple of years a technique known as faceted browsing has become something of a standard on e-commerce sites.
Faceted browsing allows users to filter data (typically product data) by groups of words that describe them (e.g.: colour, size, style, manufacturer’s name).
Amazon.com were one of the early proponents of faceted browsing and it is now such a common element of any large shopping site that it is more noticeable by its absence than by its inclusion.
This article by Mike Padilla over on Digital Web Magazine gives a clear and concise breakdown of the technique, and offers some great advice on planning faceted browsing for your a site, highlighting best practice and potential pitfalls.
Original article: http://www.digital-web.com/articles/user_interface_implementations_of_faceted_browsing/
Just as it is important to choose the proper knife when slicing-n-dicing vegetables, it is critical to prescribe a suitable user interface to support faceted filtering. Faceted filtering allows you to narrow down a large list of objects to a manageable size by applying flexible combinations of attribute filters in any order.
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If we define groups of adjectives (facets) that describe objects and allow users to filter with them, we could empower users to manipulate the information space themselves rather than oppressively imposing a fixed structure upon them. Users could flexibly select values across all facets, in any order, to view only those objects that could be described as such.
Selected screen-shots from the article

Placing an item count after each facet value effectively gives users future insight without requiring additional interaction
Original article: http://www.digital-web.com/articles/user_interface_implementations_of_faceted_browsing/