With the price of flat panel monitors continuing to drop, many people are opting to actually have two monitors. I have done this at my office, and I can tell you it is wonderful. Some may view this as unnecessary and excessive, but I can tell you that once you have tried it, it is difficult to go back to using just one monitor at a time.
One of the advantages is that you can have one program open all the time, and viewable, on one screen while you are working on the other screen. I generally have my email open to see when something new comes in, and the other one has Photoshop, Firefox, or some other program that I am frequently using.
I have mine side by side like this:
You can also get mounts that will arrange them in a top/bottom fashion as well, depending on your preference. Some mounts will do either.
Another fun thing is that you can have one single wallpaper image that is spread over the 2 monitors. Here are some sites for dual-monitor wallpapers: