iDisplay is one of those apps that you receive as a press release in your inbox and just when you're about to move on, it suddenly hits you exactly what that app does. And you go "Oh man! Yes!" In this case, iDisplay transforms your iPad or iPhone into a second monitor for your Mac.
I haven't had a chance to play with it yet (I already downloaded the software but haven't received my iPad) but I am so looking forward to using it. It works like this: you install an extension on your OS X desktop system. (A Windows version is in the works, as well.) That extension looks like a monitor to the underlying operating system, which begins to send data through the extension to any attached device. The data is sent over Wi-Fi to the iPad (or iPhone, if you're using an iPhone, as it's a universal application), which acts as an extra monitor.
It's not just a one-way connection. The on-device keyboard generates shared events for OS X, and the screen allows you to treat your second monitor as a multi-touch input device.