Duet Display is a very handy app if you need more screen real estate for your Mac. With this app you can use an iDevice (iPad or iPhone) as a second display for you Mac, all you need to do is connect via a Lightning or 30-pin cable.
Duet Display is built by ex-Apple engineers, delivering a retina display at 60 frames per second with no lag, which is pretty impressive. As long as you have a recent OS on your Mac and iPad/iPhone then Duet Display promises to be compatible with all of them.
You can download the app and find out more via duetdisplay.com