I have 2 monitors hooked up to my older computer with a video card with two monitor plugs. It's awesome. I can watch a movie while checking my e-mail on another monitor.
You can describe a scene in a movie through e-mail and watch the scene and type the e-mail on the other screen.
I think if your taking some online course at school it might be usful to have two monitors. One monitor to watch the video of the teacher speaking, and another screen to type your notes, draw diagrams, and to record the video.
I mostly use one monitor since I just go on forums on the computer.
I think some newer games are good for dual display like counter strike. One screen is for your guns, gear, and items. The second screen for playing the game, and showing your HP, and other vital stats.
For a while I had my laptop hooked up to my computer as a second monitor. Might start doing it again, seeing as how I'm taking up a lot of desktop space with many applications I have running.