The Best Answer
If you want the best app for using your iPad as a monitor, you'll want to get Air Display. It works on both Windows and Mac computers and converts your iPad into a second monitor and extends your desktop onto the screen. It's very easy to set up and even works on iPhones and iPods (not sure who would do that though). Air Display not only gives you a wireless computer screen extender, but also works as a tablet input device. You can use your finger to draw directly on the screen’s surface (good for calculator app, painting tool, or music app). Maxivista, their competitor, doesn't offer the ability interact with the touchscreen or play video at full screen. If they're both $9.99, Air Display is the obvious choice.
If you use Windows, you can use MaxiVista. It's the same price as Air Display, but it's built specifically for Windows Vista/7 users.