It seems like that would be pretty straightforward to add to any filesystem that already supports metadata. I'm sure Windows has something similar, but on MacO

It seems like that would be pretty straightforward to add to any filesystem that already supports metadata. I'm sure Windows has something similar, but on MacOS X.5 (a feature borrowed from BeOS, IIRC), you can create a Smart Folder that's basically a canned query for files matching a very large set of criteria. I'm not sure what the API is for extending the criteria, but presumably adding tags wouldn't be all that difficult.