Just got back from attending the kickoff and final demos of the weekend-long Music Hack Day SF May 2014 held at the headquarters of GitHub in San Francisco. It was tremendously inspirational to be around such focused energy, and the API demos during the kickoff were educational (though very brief).
In the end 41 teams formed, pitched, developed their products in 24 hours, and demoed them. Quality was mixed, though even the unfinished and buggy ones usually had innovative ideas behind them. The APIs used were the usual suspects such as Spotify, RDIO, SoundCloud, EchoNest, GraceNotes and Twitter, with the Leap Motion gesture tracker being a surprisingly popular (and impressive) choice of input device.
The GitHub space was an inspiration in itself, as captured in this recent Wired article on it.