The MusicApiCollection C# SDK by BigGranu provides access to an assortment of music-related APIs including Acoustid, Cover Art Archive, Discogs, fanart.tv TV, Freedb / CDDB, Gracenote, MusicBrainz, and TheAudioDB.
Last.fm besides being one of the most popular online radio and music communities, also has one of the most popular online music APIs. Possibly the most popular. Each month we continue to see a variety of new music-themed mashups added to our directory built on the Last.fm API. In the first three weeks of this month alone there have been 5 more Last.fm mashups added. That means that out of 231 music mashups at least 84 use the Last.fm API. And not that developers don't have other options given that there are 44 music APIs to choose from.
2010 is not yet half over and already Music Hack Day is hitting its third city of the year: San Francisco. On May 15th and 16th, scores of hackers will convene at the Automatic Lounge for a 48 hour hack fest themed around all things music.