A web app that plays music (could just be youtube videos), and makes smart ratings and playlists for you based on how long you listen to your songs. The idea is that if you listen to a song for 20 seconds, you don't like it as much as if you listen to it for 2:30. From this data on how much you like songs, you could even get into better predicting of which songs you should try (like Pandora does).