Echowrap is a Ruby gem that supports 100% of the v4 Echonest API. I put a lot of work into building a nice gem for Ruby that will hopefully help support more audio apps in the Ruby community. This wrapper was built using the Twitter api gem as a reference, so instead of dynamically calling the API each Echonest API method is coded and documented. Also instead of returning responses as data hashes, I created objects which makes it easier and nice to access the data instead of traversing hash trees. Finally, it uses Faraday under the covers so you can use other http client adapters (Excon, Typhoeus, Patron, EventMachine).
Have a look! http://echowrap.com