Here's an interesting mouse. The Rocket Mouse is a hand held (actually held in your hand instead of resting on your desk) mouse where the thumb is used to press the buttons and roll a track ball. According to the site, this mouse "frees your hand from the desk and allows a relaxed ergonomic posture, relieving arm, wrist, and back pain." It is an interesting idea. I'm not sure how well it would work in conjunction with a keyboard, but for mouse intensive activities it could provide a break for your fingers.