As part of my final year of studies at university I’m working on building a metadata filesystem, called “MDFS” or “MetaData FileSystem”. Creative naming don’t you think? The project is using FUSE to create the filesystem interface and PostgreSQL is used to store the data in the back end. Its in its early phases, but you can check it out here at a Trac instance I’ve created: http://mdfs.pasamio.com/.