Last.fm: Provides music playlist management. Simple REST-like api returns plain text, XML or RSS data. It’s powered by the Audioscrobbler.net API. For more see here.
Tagyu: A ‘tag recommendation engine’. From the site: “Tagyu compares what you’re writing to what other people around the web are writing. It looks at how they’ve tagged their content and uses that information to give you some ideas about how to tag yours”.