Dunno of any, but the CLI tools TMSU, Tocc, and Tagistant all create their own file systems (using symlinks) inside your drive's existing one that can then be accessed by other software. Pretty handy.
Software that stores it's own database of tags, as a way to consistently tag all sorts of files, no matter what metadata they support. I've tried some before for organising images and videos, and am currently trying some for organising music samples and sound effects.
I generally just use these tags:
TIT2 / titleTPE1 / artistTALB / albumTPE2 / albumartist - Quod Libet labels this performer. Usually only use if there are multiple artists, software seems to sort stuff right otherwise.TYER / dateTPOS / discnumber - If there are multiple.TCON / genre - I have set genres I use, and I also place the country and sometimes city in here.TRCK / tracknumberOnes that seem neat that I should use:
TSST / discsubtitleTLAN / language - Fill with three-letter language code from ISO-639-2mul for multiple, zxx for none.TSOP / artistsortTSO2 / albumartistsort - Starting to try these two 'sort' tags now though.TSOT / titlesortTSOA / albumsortWOAF / ? - Track website.WOAR / ? - Track artist website. Well supported across file types.WOAS / ? - Source website.Maybe notable ones I don't bother with are:
TCOM / composerTSOC / composersortorderMVIN / movementnumber