I use dual monitors--wish I had Quadruple monitors, lol. What a huge time saver! It is so convenient when creating my webpages. I just click 'save' on my text editor on one monitor and refresh my browser in the other monitor and almost instantly see the modified webpage, thereby eliminating switching back & forth between windows on one monitor. Also great for many other apps.
With my ATI Hydravision, I can have active windows stretch across both monitors I wanted to or only one. Here's a couple of links for more multi-monitor information:
Dual Monitor Setup Saves Time
Multi-Monitor FAQ