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