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.
Something awesome is happening in the world of music and the web. But first, some primer.
Rdio is one of the few music subscription services that do a tremendous amount of things right, including having its robust Rdio API. For $5 a month, you can get all of the music you want, have collaborative playlists, share content to Facebook and Twitter, and keep up with what your friends are listening to. For $10, you can do all that and store the music to your phone for offline or higher quality enjoyment.
Seevl is a new kind of music discovery engine. It allows you to find how artists are similar to each other and which artists are most similar to each other, among other things. It also allows you to comment on the notes about artists within the database. This is all well and good, but better yet, it has the Seevl API, allowing developers to integrate Seevi into their own applications.