I'm a fan of Logitech optical mice. My current favorite is an MX Revolution wireless. The tracking is excellent, the batteries last a while, and the scroll wheel has a better feel than that on any other mouse I've tried. Plus it has this switchable mode where the scroll wheel spins freely up or down, so you can give it a flick and quickly scroll through a document or web page. And the wireless dongle is small enough that it's easy to switch it from my Mac to my PC when I have to dial in to work.

The only down side is that the thumb control requires too much force. I hold the mouse with light fingertip pressure, but I have to squeeze it firmly to operate the thumb control.