iDisplay Lite is an interesting app that lets you use iOS devices as displays. To get this to work, you need to install two applications. One is installed on your iOS or Android device and the other one on your Mac or Windows PC. The app that you install on your Mac, you can get either on the Mac App Store or from the developer's website. The one of the App Store is called Lite, but it seems to have full functionality. It was just named that way for the App Store.
I tested this app with an old iPhone 3G and my Macbook. When both apps connected, I was shown a list of running apps on the iPhone. When I selected an App, it was shown on my iPhone's screen. And I could control it by tapping on the screen. If a running app has multiple active instances, you will be able to choose which one you want to select.
All in all, the application works well, and it even displays a list of IP addresses and ports for when the automatic connection doesn't work. I was expecting an app that would let me use my iOS device as a secondary device, to which I could drag app windows. Although not what I expected, this app could work for other users.
The iOS app costs $4.99.