At the London 2011 Music Hack Day we set about doing something interesting with the Echo Nest API and Spotify.
What came out was an interesting variation on our CTRL installation, where users can choose any track from Spotify and interact with it via 2 iPhones, adding effects and synth lines in realtime.
The Echo Nest is an online data service, providing all sorts of metadata about millions of songs. Simply ping it with the title and artist of a song (or a snippet of audio) and it will return the relevant information, including BPM, key, current artist popularity and much more. These first two bit of information were of most interest to us for this hack, as we used the BPM to tempo-sync all of the effects, and the key information to make the synthesizer sound great with the track.
Check out the video of the finished hack in action!