SemFS is a semantic file system based on "RDF":http://www.w3.org/TR/rdf-primer/. On Windows, it can be mounted as a WebDAV drive. For Linux, SemFS can be used as a user level file system via FUSE. SemFs currently supports tagging of files and browsing according to various ontologies.
The Linux version can be downloaded here. The Windows version is currently being rewritten.
Bloehdorn, Stephan; Görlitz, Olaf; Schenk, Simon; Völkel, Max (2006): TagFS - Tag Semantics for Hierarchical File Systems. In: 6th International Conference on Knowledge Management (I-KNOW'06), Special Track on Advanced Semantic Technologies. Graz, Austria: Goerlitz2006TBS
Görlitz, Olaf; Schenk, Simon; Staab, Steffen (2006): TagFs - Bringing Semantic Metadata to the Filesystem.