App that can display least used apps

Although it might not be exactly what you're looking for, it's worth trying as it's built into your iOS device. Just hit the home button twice then swipe left with three fingers as many times as the device will allow. Each page is it's own app. The higher the number the longest it has been since it was last opened. This process won't tell you about apps you haven't opened or you've previously closed in the app switcher. to close an app you can flick down once then one finger double tap.