Tom Coates has a great post on bubble-up folksonomies–using tags to augment conceptual hierarchies. The example he gives involves tagging songs (in his Phonetags project) and using those tags to understand broader categories like album and artist.

…because you have a semantic understanding of the relationship between concepts like a ’song’, an ‘album’ and an ‘artist’ you can allow people to drill-down or move up through various hierarchies of data and track the changes in an artist’s style over time. For me, this is a pretty compelling argument that understanding semantic relationships between concepts makes folksonomic tagging even more exciting, rather than less so, and may indicate a changing role for librarians towards owning formal conceptual relationships rather than descriptive, evocative metadata. But that’s a post for another time. (My emphasis)

I like this idea for a couple of reasons. First, it recognizes the value of some semantics in the system (that is, the humans-at-both-ends-of-the-rope approach would be insufficient on its own). Second, it solves a real problem–helping people find good music that they’ll like.

And In the comments Kevin Marks says that Technorati is using the bubble-up approach to recommend blogs by tag (rather than just posts).