If you're listening to music while using your phone or tablet for something else, you don't always want to stop what you're doing to fiddle with the song. If you're in a game, for example, you may have to exit to the home screen just to get to your music controls, but SidePlayer offers an alternative. It pops a small control panel out from the edge of the screen when you need it, and hides it when you don't.
SidePlayer ties into various music players like Google Play Music, PowerAmp, n7 player, and several OEM music apps. Just drag in from the right edge of the screen and the controls appear. Drag back, and they recede. This works in any app, even a full-screen game. The free version of the app is limited to this basic functionality, but the $1.75 pro edition includes themes, metadata display, more layouts, active area positioning, and more.
This app doesn't make a lot of sense for devices that can show the notification shade in full-screen apps (like Samsung devices and stock KitKat), but it could be great for others. You can give it a shot for free, so why not?