Annie,
any USB mouse you've used before will work, as will a bluetooth mouse. Here is my thing about it though.
If you're like me and you use more than one button on the mouse, think about this combo: Logitech G5 MX lasermouse and a small mac app called USB Overdrive. Problem with Logitech is that they dont have good mouse drivers. The mouse will work, but buttons 5-9 will not work, nor will the scroll wheels or things like that. With the USB Overdrive, they do. Now I've got the scroll, page forward & back, expose, and others mapped all to my mouse. Any usb mouse will work with USB overdrive.
Bluetooth, heres the problem. You'll only get basic functionality out of any bluetooth mouse except for the mightymouse. No bluetooth mice that I know of have drivers written for the mac to let you use the other buttons. Maybe the MS bluetooth Intellimouse explorer version does, but Im not sure. For the record the MS mouse is very comfortable.