Which Version of Air Display?

Air Display 3 is the latest and greatest.

Currently only for iOS 7 & 8.

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.

The Original Air Display.

Although no longer available for purchase, it is still supported.