I agree with a lot of people about Spaces being the alternative to multiple monitors. I usually use my laptop hooked up to a display and keyboard so I do not use Spaces. But, when I am just on the built in screen, it is a godsend. Especially when doing computer simulations. I can have the code in one display, the terminal and some other information in the next, then I can have the web browser on the man and help pages in the next. After that, I have my electronic lab notebook. Finally, I have one with email and personal stuff.

It is also great in class because I can have my notes on one and then my web browser on the other. If the professor is walking by, I can switch.

Expose is by and large, the greatest mac feature ever. If absolutely love it and it is essential to me. If I ever need to move to windows, an expose clone will be sorely needed. (on a side, the next best feature for me is QuickView. Use it every day!)