Last.fm Lib .Net is a code library for the Last.fm API written in VB.Net 2008, but it can also be used with C# and ASP.NET. It provides a complete implementation of versions 1.0 and 2.0 of the API, the Radio API, and the Audioscrobbler 1.2.1 protocol.
The Independent Online Distribution Alliance (IODA) has just announced its IODA Open API, currently in beta. The API offers many of the typical resources that you would expect from a music metadata service, with a few extra goodies like these album attributes: UPC code and primary label. Comparing album API results between IODA Open and iTunes, IODA is packing a lot more information into the response.
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.
Rumblefish, champion of what is good and just and legally musical, will help you change your tune for the better. This company has produced a music licensing API so that you can add copyrighted songs to your YouTube videos. Of course the concept can be applied more widely and liberally. Rumblefish has even devised custom license ‘portals’ that present licensing options based on the intended use of the music. This implies that the agreement and cost of licensing a track could be more expensive depending on where and how it is used.