Air Display 2
$10. Current App Store rating: 3 stars
With Air Display 2 and a free desktop utility (downloaded from getairdisplay.com), you can use your iPhone, iPod touch, or more likely, an iPad as a second display for a Windows or Mac computer. (The two devices must be on the same Wi-Fi network.) If you're into multiple monitors, especially when used as one extended desktop, this app is worth 10 bucks. You can also mirror your screen on up to four iPads simultaneously. (This feature only works with Macs). But there's one big potential downside: In our tests on public Wi-Fi networks, we couldn't always get the app to work.