I switched from mice to trackpads and trackballs years ago after my wrist started getting all janky on me after a few hours a day of work. That's because seesaw mouses like SmartFish Technologies' ErgoMotion laser mouse didn't exist back then.
The mouse, which just launched officially, isn't static like most, but rather has a Y axis and X axis pivoting motion so it fits more naturally in your hand as it moves. When you push a mouse forward the geometry of your hand is different than when you pull it back. The mouse is designed to shift with your hand's geometry. Neat trick.
Made by the same company that brought us the Pro:Motion family of moving keyboards, the ErgoMotion laser mouse goes for about $50 and works with Windows or Mac OS X and connects via a wireless USB dongle (included) so the wires don't get in the way and force the mouse one way or another. It's also designed for left hand or right hand use, something many ergonomic mice don't feature.
I haven't had a chance to try an ErgoMotion mouse yet, but it certainly looks comfortable. Which reminds me, I should look getting into some ergonomic pants for my Thanksgiving feast.