The Kensington Expert Mouse Optical Trackball is a choice for the man/woman with ergonomics in mind. If you are tired of throwing a mouse around over you desktop, I would recommend a trackball. The trackball is also great for small places, as it doesn't have to move.
The Expert Mouse has a big and sturdy construction, and fits your hand like a glove. It has a big ball, and compared to other trackballs this gives a much more pleasant experience. It has a fantastic scroll wheel, configured out of the box to scroll up and down. You can scroll sideways by holding shift, as with every other mouse/trackball.
The Expert mouse has 4 configurable buttons, and is easily configured in the Trackball Works program from Kensington. (This is only needed to configure the remaining two buttons). Out of the box the trackball has left click on down left, and right click on down right buttons.
With the TrackballWorks program you can sett any command or keystroke to your remaining buttons. You can also program keystrokes for pushing two buttons at once. This opens up for a new computing experience were you can do more without moving your hand from your mouse.
I've had my Trackball for 4 years now, and it still works like a charm. The only sign of age is the wearing on the left click button. The buttons, the scroll wheel, the ball itself has never let me down in 4 years. The only problem i have, and it's a minor one, is that dirt usually gets under the ball, making it move slower. This is very easily cleaned up, so as I said, a very minor issue.
The trackball has saved up my desktop-space and my arm movements - I'm never going back to a mouse again!