ProgrammableWeb lists many popular Music APIs in the directory including Last.fm, 7digital and Spotify Metadata. This post features a few really creative, entertaining mashups that use a few popular Music APIs.
The legendary Billboard charts are now open for mixing, not by DJs, but by programmers. Billboard, a service of Nielsen Business Media, has a new API that lets developers search and read information from all the Billboard historical charts. The initial offering is its Chart service and later on Billboard will "roll out the Artist and Track services to add even more depth and perspective." the API is a simple REST interface that developers can send HTTP GET requests to in a familiar service/version/resource fashion. We've added a new Billboard API profile with details.