The Echo Nest Ruby Library echonest-ruby-api is a Ruby wrapper around the Echo Nest API. With the library, developers have a way to access different methods for making calls to the API using Ruby on Rails.
For staying up to date with band tour dates, live venue information and more, Pollstar has it covered. It's a trade publication that covers music industry information on a worldwide scale. What's pretty cool is that Pollstar provides an API that allows third party developers access to the full event database, enabling them to include Pollstar data on other websites and applications.
The most popular API, in terms of mashup count, is far and away the Google Maps API, which accounts for 41% of all mashups. But when it comes to the most popular pair of APIs, Flickr and YouTube mashups are the most common. Not to be outdone, Google Maps joins Flickr in a near second place.