The Echo Nest web music API is fairly awe inspiring. For example, they have a web service to detect the sections of a song — bridge, chorus, etc. — given only an MP3. What it amounts to is a bot capable of doing semantic analysis of a sound file.
One thing you could do with it is to distinguish spoken word from music files. A hosting service could then automatically reject music but allow talk, which would control its legal liability.
All in all it’s a radical level of power for the kind of lightweight apps that use third party web services.
One complaint so far: the web service returns XML, but the XML isn’t in a namespace. This will cause pain down the road, and it can easily be alleviated by adding an xmlns=”http://echonest” attribute to the root element.