Google Play Music's desktop web app has a cool visualization you can enable when playing a song, but it only works in Chrome. Mouse over the small album art thumbnail at the bottom of the window and click "Particles" to enable the visualization. You can disable the visualization by pressing Esc or clicking anywhere inside the animation pane. To enable it again, just click the album art thumbnail.
If you don't like the particles visualizer, you can always mouse over the album art thumbnail and pick the album art visualizer, which is more subtle. Google Play Music remembers your last pick.