Use your iPad as a second monitor for your Windows PC, YouTube video.
I can't believe this came first for Windows!
Update: aha, you can do it on Mac too. TTL points to AirDisplay.
... I am trying it on my Mac Pro. Apart from the screen updating being slightly slower than we're used to, it works well, so now I have three monitors on this durn thing! (A bit superflous to most use, all right.) I think I'll try it on my "Airbook", where it would do much more good. (Some people can't get it to work on their laptop.)
Update: works great on the Macbook Air also. And on that one it expands screen space by almost double. And in one compact package which can also be used for many other things. Not bad for traveling.
And it has been pointed out it's about the highest quality display you can get (for a laptop).
(Image from linked article.)