Tomahawk is an odd tool, but perhaps perfectly suited to how most people consume music these days. While it's a desktop player and will work with local music, it can also connect to a number of streaming services, from YouTube to Spotify.
It's also got an online component, Toma.hk, though it's a stand-alone one. And now, there's an API for it, like any self-respecting web service should have.
Toma.hk is a great tool now when everyone has music in different places. WIth it, you can share a track and be sure that whoever stumbles onto it will have a way of listening to it on whatever service they use, SoundCloud, Deezer and all the rest.
With the new API, you can embed the Toma.hk player anywhere you want, making it possible to build a music app that actually works for everyone, rather than having to pay millions of dollars to license the music yourself.
It's not a huge API, but that's the point, all the basics are there, you can play music, get artist data and everything else from a multitude of places without having to work with half a dozen APIs to do it.