The MusicBrainz (libmusicbrainz) C/C++ Library by MusicBrainz is a client library for developers. Using this library, developers can add MusicBrainz look-up capabilities to their applications. The MusicBrainz database contains a huge amount of music metadata, which is maintained by the MusicBrainz community.
APIs tend to go wherever the data is, so with more data than ever showing up in Hadoop, it stands to reason that APIs would soon follow. At the Strata Conference and Hadoop World 2013 event today, Cloudera announced a Cloudera Connect: Innovators program under which developers are invited to build applications that invoke an API that Cloudera in partnership with DataBricks has created for its distribution of Hadoop.
The InternetDJ API opens this electronic music site to your app. Since 1997 (not a typo!) composers and fans have been able to share and mix music and videos. According to the API's website, requests are limited to 5,000 a day, responses are in JSON and an auth key is required.
The Federal Register is touted as "The Daily Journal Of The United States Government". Its website, federalregister.gov, allows access to the documents published there, which include proposed rules, final rules, public notices, and Presidential actions. It's a pretty interesting and useful look into what the US Government is doing, and how it might affect your life. Amazingly enough, it has a full-featured API, which is a truly open source project, to allow access to the Federal Register within applications.