A many buttoned trackball made by Microsoft.
As much as I dislike Microsoft's software they do make some nice pieces of hardware, like their mice and trackballs.
I used to use a
Microsoft Wheel Mouse Optical (its basicly the same as the Microsoft IntelliMouse Explorer without the two side buttons) but then I started using my laptop while sitting in places other than my desk and it didn't work so well on the leg of my jeans and my wrist started hurting (urggg RSI). The optical mouse was the best mouse I've ever used, no gunk, light, great sensitivity, works well under Linux and it glows red! With this in mind I thought that I'd get an optical trackball.
After looking around different websites I had narrowed it down to either the MS Trackball Explorer or the Logitech equivalent. The main differences between the two are the the MS one has two extra buttons (they both have a wheel) and you use your index finger to move the ball, the Logitech one uses your thumb to move the ball. Deciding that my thumb wasn't made to move in certain directions (moving the ball down-right just felt wrong) I went with the MS trackball.
The Trackball Explorer is quite big and odd looking, with it many buttons. Its got two buttons with a wheel in the middle on the left side where your thumb goes. The trackball is in the middle where your index and middle fingers naturally fall. on the far right, next to the trackball are two more buttons, placed just under your ring finger and pinkie.
It initially feels quite odd to use. Using your thumb to operate the first and second buttons as well as the wheel feels quite odd at first, although I found that after a few hours use it became quite natural. The trackball is big, red and shiny with a sort of dotted pattern on it, it lies in the middle of a red glowing ring which looks very cool in the dark. Using your index and middle fingers its very easy to use and after a few hours you can flick the pointer from one side of the screen to other with ease, as well as being able to make small, very acurate movements. The other two buttons on the right I've yet to use (you can set them up for custom functions like forward and back in IE using the supplied software, but I don't use windows so I've yet to program them under linux and X) but they seem quite easy to click with your ring and little fingers.
Overall I'm very, very impressed with this trackball. I've found it very comfortable and easy to use and very quick to get used to. My only gripe with it is having to lift my thumb over the wheel to hit the second button, but its starting not to bother me so much.
For more information look at: http://www.microsoft.com/hardware/mouse/te_info.asp