One of the things I was most excited about with my MacBook Pro was the ability to run Parallels. People ask "if you like OS X so much why are you excited to be able to run other OSs?" Here's one reason.
In my distributed applications class, I have my students set up and manage their own Linux server. For some of them it's the first time they've been root. They have to install jBoss, Axis, and other fun things before they can complete the assignments.
As a consequence, I end up working on a Linux machine quite a bit to make sure all the assignment works. Having one run in a window on my local machine is pretty handy. But that's not the best part.
Because your hard drive in Parallels is just a file on the OS X file system, you can copy it, burn it to a DVD, and do anything else you might do with a file. Once I had Fedora Core 3 loaded into Parallels and the initial configuration done to Fedora (including installing Emacs, creating users, etc., I burnt a DVD of the image and set it aside. Then I performed the complete set up for the 462 class and burnt another DVD with that image.
Now, rather than redoing that configuration each time I need a clean copy, I can just copy the image file from the DVD and start it up fresh. That saves a lot of time. Of course, I can have multiple images in different states and fire them up at will.
If you use Window's a lot, doing this of the hard drive image right after you load XP will ensure you don't have to ever load XP again--just copy the image over. This isn't something that will surprise anyone familiar with virtualization technologies, but still, it's fun to see it work.