SHAPE GmbH has iDisplay (iTunes Link) for the iPhone, iPod Touch, and iPad in the iTunes App Store for FREE (regularly $9.99) for a limited time.

Turn your iPad, iPhone or iPod touch into additional display for your Mac or Windows computer. Increase your personal productivity by extending your computer screen! Perfect tool for multitaskers, iDisplay lets you use your iPad, iPhone or iPod touch as interactive wireless display to give you more screen real estate.