Music Center (free) is a very cool little app that lets you see your iOS music library from your lock screen or inside any app. It's a notification center widget, and it works quite well. You load the app, and you can select lists by song, album, artist, and playlists. Then go to your Today notifications and scroll down to the edit control to add the widget to your screen. That's all there is to it.
I wasn't sure how useful Music Center would be, but I find I am using it frequently. I especially like the access from the lock screen to play music via Bluetooth while I am driving. It's quick and easy, but of course Siri can also select music by using your voice. Since the notification feature is a quick pull-down from anywhere in iOS, it's also useful if you are in the middle of using another app.
Apple might pull this widget if it is found to be in violation of any developer rules, but so far, so good. Music Center is clean and useful, but note that you can't use iTunes Match or search since widgets don't get to use the keyboard.
A pro version, which is a US$ 0.99 in-app purchase, gives you scrolling instead of buttons, but I find the free version meets my needs.
Music Center requires iOS 8 or later, and it's optimized for the iPhone 5 and 6 phones. It's a universal app, so it will run on iPads too.