I can't remember what is is, but I'm pretty sure there's some program that uses an interface similar to Napster/Limewire, but in the background it searches YouTube for a song, downloads the video, and extracts the mp3.
A service like this is bound to yield inconsistent quality, and YouTube will constantly be removing videos, but I suppose it could be made to work.
Yes, there's been tons of YT interfaces over the years, but for some reason I always go back to youtube.com. I didn't know about downloading the mp3. I think that'd be against YT's terms, although I would like to be able to access a vid's soundtrack and not be forced to dl the video.
The innovation is really about labeling YT vids with the correct metadata automatically, rather than manually. In fact, YT should be connecting its music and artists to Musicbrainz already upon upload. It's about connecting artists and songs on yt to their corresponding pages on MB,
So you could take your local mp3 collection, tag it, save as a playlist, delete all the files and then access all the music on site from YT via the playlist.
I think the best way to operate such a system without YT involvement would be to have a browser plugin that fingerprints and tags YT music vids as they're played in the browser, there'd have to be some interaction, but as a distributed effort, you'd gradually build up a yt-mb database.
A service like this is bound to yield inconsistent quality, and YouTube will constantly be removing videos, but I suppose it could be made to work.