I bought a new mouse over the weekend and it's been working out great. It is a Trackball mouse, specifically a Kensington Expert Mouse. The Trackball is a little larger then my previous mouse (logitech) and this one has four buttons instead of two.

The top left button enables a scroll and the top right button goes back. These can be pretty useful features. I will keep you informed of how this mouse is working out, so far I'm pretty impressed with its performance.