It's not just the iPad which can become a second monitor. You can also convert your Kindle eReader into a second display as well. The e-ink screen is easier on the eyes for reading documents and web pages. Now if only it were larger...
Tiny Apps posts the full directions for this transformation, using a Kindle DX and a Mac. PCWorld has Windows instructions.
The first thing you'll need to do though is to jailbreak your Kindle, by installing hawhill's Kindle VNC viewer, and setting up the network connection between the Kindle and your computer, which are connected by USB.