If you're in the entertainment industry or you think a 'best songs for x' will work in your niche, the Spotify API can be very useful.
This is also a very user friendly API, and a good introduction into how to use them.
Part 1: Making a Spotify web App
Part 2: How to get an access token
Part 3: How to get Spotify ID's
Part 4: How to get Track ID's