Echonest are some cool people. Their earlier APIs enabled the illustrious although sadly now defunct http://www.donkdj.com http://www.donkdj.com which was done

Echonest are some cool people. Their earlier APIs enabled the illustrious although sadly now defunct http://www.donkdj.com http://www.donkdj.com which was done by a classmate of mine as a project for a Generative Creativity course we did at uni. Looks like their research has taken them a lot further!