Stereomood is a streaming music service that gives recommendations. To start, it uses a list of moods, each of which have an associated playlist. While playing songs, users can like or ban songs, much like Last.fm or Pandora. The Stereomood API allows full access to the site's functionality to developers, allowing developers to make fully usable clients on any platform.
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.
The Punkrock.org API uses POST and GET calls that allow a user of your app to access their own user info on Punkrock.org, including friends, mail, notifications and allows them to upload photos. Requesting an API key is required for access to services.