I love Rdio. All the music I want to hear (that happens to be available in their library) for $10 a month. It gets better. I can download the songs to the mobile Rdio apps for offline listening. Even better still, there's an Rdio API and it's pretty rich, as described by Rdio's Ian McKellar in the video below.
Rap enthusiasm site, Rap Genius has gone through the impressive tasks of combing rap and Google Maps. Rap Genius' Rap Map is a Google Map-based visual almanac of all things rap. You can scan an area and view rap landmarks and places mentioned or referenced in some famous rap lyrics.
Vadio is building a business off the YouTube API. The company also has its own technology for translating a list of songs into music video playlists no matter where the videos are stored. Vadio has already seen interest from radio stations, because the product is more appealing than simple audio and increases listener engagement.