I've been hacking a lot on metadata with great success. My application does nothing, but it does it very well. Really, I'm very proud I have code structured to work when I put real logic in the methods.
I selected some existing schemas that I think the metadata db should know out of the box: Dublin Core (documents), FOAF (people), EXIF (photos), MusicBrainz (music), IMDB (movie), and a few others. Absent is a schema for the filesystem. I think I'm the only person thinking about metadata as an alternate index to the filesystem. I need to create a schema that represents stat, mime-type, and nautilus thumbnails + emblem/keywords.
PS. I've switched from my usual britpop selection to the darker tones of Placebo, British Sea Power and Interpol. I feel more productive. Don't fall to sleep with Placebo playing in a loop; I woke up feeling like I had spent the evening on a drinking binge.