Which Version of Air Display?
Air Display 3 is the latest and greatest.
Currently only for iOS 8 & 9, but we plan to support iOS 7 again very soon.
Requires free host software on Mac 10.8 OS X and up (but the latest Mac host can still connect to older clients on iOS, Android, Mac, and Windows.)
Air Display 2 is available for iOS, Android, Mac, and Windows.
Connects to host on Windows as well as Mac.
NOTE: Air Display 2 for iOS is, hopefully temporarily, unavailable. We have removed the app from the App Store until we can resolve a crash on iOS 9.
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.