Last.fm collection of C++ libraries by mhx includes algorithms, abstractions, configuration, object oriented shared object loading, template meta programming, and XML parser among other resources that aim to facilitate API interaction. With these libraries, users can access albums, artists, and playlists. Installation instructions available.
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.