Mo' Faceted Classification
Faceted classification is quietly taking off. Here's a quick overview of some implementations.
The original browsing tool for faceted classification was Flamenco, a research project at Berkeley university, which happens to also be the home of much of the more interesting research on how humans categorize the world (think Lakoff). But anyway.
Further research is being done in universities, often focussing on trying to implement a system using faceted classification for access to information. Facet project at University of Glamorgan for example. Or FATMKS at UCL.
Commercial implementations exist as well: bpallen (responsive engineers) and Endeca (hidden behind an almost inpenetrable wall of marketeese) both provide systems for browsing information with FC. They don't seem much better than the Flamenco system, interface-wise, though.
In a category of its own we have Facetmap: it's interface is excellent and evolves fast. Facetmap is the work of one man.
Faceted classification (FC) is also being taught. This homework paper (PDF) is an excellent tutorial for FC in itself - check it out. Really. If you're still confused, here's an excellent Berkeley slideshow onclassification systems. More papers are yours if you keep digging: Experiences with a Faceted Classification Scheme in a Large Reusable Software Library (in short: faceted classification alone isn't enough). But we were talking about implementations.
I believe implementations will quickly reach an acceptable level of maturity. The interesting bit comes when real life experiences start coming back, and when easy availability of tools allows more experimenting. I think faceted classification theory will have to become more rich, and that richness will come from the experiences with interfaces. And not just browsing interfaces, creating faceted metadata (or any kind of metadata really) is a huge challenge, and tools can really help there as well. And then distributed metadata. Anyway, my point is: tool availability is coming, and that's good because that will allow us to experiment and then refine the theory. I'm gonna shut up now.