Which Version of Air Display?
Air Display 3 is the latest and greatest.
For iOS 7 or later.
Requires free host software on Mac OS X (10.8 or later).
Air Display 2 is available for iOS, Android, Mac, and Windows.
Connects to host on Windows as well as Mac.
The Original Air Display.
Although no longer available for new purchases, it is still available for re-download. See notes on re-downloading a purchased app in Apple’s Knowledge Base.