Actually I use Rdio, Not Spotify, Your Link Is No Good To Me
liisten.com
liisten.com
Their approach: your library becomes (title, artist, album) tuples, and then songs are found at whatever provider you use when you want to listen.
The devs are mostly ex-Amarok and everything is on GitHub: https://github.com/tomahawk-player.
http://forums.musicbrainz.org/viewtopic.php?id=3463
Ultimately unless someone uses MB and/or other data sources to create an aggregated 'music translation' service - whether it's entirely offline translation, translation-on-demand, or some kind of mix - I'm not sure how this is going to happen with so many proprietary platforms.
It's called http://trackpile.com and it allows easy sharing from Spotify to Rdio, and vice versa.
Would love if you guys could check it out!
http://liisten.com/i-dont-use-the-same-music-service-as-you-...