David Weinberger has blogged about North Carolina State University's new library system that uses faceted classification to aid searchers: http://www.hyperorg.com/blogger/mtarchive/faceted_classification_at_work.html
It sounds pretty intriguing, kind of like a related articles link but actually far more difficult to describe from a data modelling perspective as the faceted categories has only a slight obvious relationship to the passing matter. From what he says it sounds like Endeca are the people leading the way with this form of categorisation/search.