iOS: Music Center is a widget for iOS 8 that lets you browse your entire music library from the Notification Center. Once it's enabled, you can start playing music without having to open and browse through the Music app.
To play a song, just pull down Notification Center, tap the album or song you want, and it'll start playing right away. That's it, Music Center doesn't have a ton of options or settings, but it does what it tries to do pretty well. If you've ever wanted easy access to your whole music library from anywhere, Music Center's a good place to start.