Leaftag - ChippyWiki
This is exactly what I had in mind a few months back. Slap an F-Spot like interface where you can easily narrow down items and you've got a really nice application that can titilate the wet corners of the mind.
Think del.icio.us applied to your files on the desktop. Add to that an F-Spot interface instead of the current Gnome file manager. It would change the way you deal with your files. Add smart search folders via the beagle backend. Do a tag team with the nat friedman's dashboard project. That would be really sweet.