Shows the song you're playing as slow-moving full-screen artwork, with controls that stay out of the way.
This page talks to Spotify directly from your browser. You need your own free Spotify developer app (one-time, about two minutes):
The client ID is stored only in this browser. No secret is needed. Spotify currently requires the developer app's owner to have Premium, and play/pause/skip/volume need Premium for whoever signs in; without it the art still works.
Try the demo to see the effect without signing in.