The nice thing about using an "extended desktop" - two monitors connected to one computer - is that you can multitask with multiple windows across a lot more screen real estate. We run our email client and instant messenger on one monitor, and then any other applications we're working in on another, to reduce communication interruptions and the need to cycle through open windows.
Many newer computers support extended desktop, and it's possible to enable it through the operating system. Apple's iBook, iMac and eMac shipped with video cards that support it, but no documentation on how to enable it. This little article tells you how to turn on extended desktop on the iBook, iMac and eMac.
Disclaimer: We used this hack here at Lifehacker HQ to enable extended desktop on our G3 iBook not too long ago, so we're living proof it works. However, this hack is only for the particularly adventurous and geeky, as it involves making changes to firmware settings. If you don't follow the directions to the letter, irreversable damage can be done to your computer.