This is a Spotify App that plays songs from a playlist in order proportional to their popularity.
Place files into ~/spotify/smartlist, and then search for
spotify:app:smartlist in Spotify.
Note that this is an unofficial Spotify app and not available in their app
store. When adding to favorites, it will only remain for the current session of
the Spotify application, and must be re-added via spotify:app:smartlist upon
restart. This and the directory limitation are imposed on all unofficial apps.