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Air Display is a useful utility that turns the iPad into a second monitor for any Mac or Windows PC. It connects to the PC wirelessly via Wi-Fi, and makes good use of the iPad's Retina Display. The app has not been updated to fully support the Retina Display, but already renders the display in brilliance that makes a desktop monitor look downright bad.
Note that Air Display is also available for some Android tablets, and in my tests is just as good as on the iPad.