Another great hack created during Musichackday in London was the iPhone Music Visualiser by Matt Biddulph and George J Cook. It’s a pretty cool music visualizer for the iPhone that let’s you browse tracks on SoundCloud (using our API) and then launch a custom visualiser for a track of your choosing on your iPhone.
The visualiser is actually sending the track over to Echonest, a great audio analysis web service, and uses that data to drive the cocos2d-based visualisation. Check out the video to the right to see it in action! Note for example how the particle systems grow bigger once the track loudness goes up.
The project is open-source so feel free to go and fork it for your own needs.