- The Echo Nest have an API that takes seed artists and provides a playlist of recommended tracks, with JSONP output.
- Spotify have an API that takes an artist name and track title, and searches for the track in their catalogue. They've added the "Access-Control-Allow-Origin: *" header, so it can be called cross-domain.
- Freebase provides their Suggest jQuery plugin, which can autocomplete music artist names.
Which means it's become pretty easy to put together an app that takes a list of artists provided by the user, generates a playlist and links the tracks to Spotify. Like this: ReCo.