I use a Kensington, myself.  You can get four... :)

http://www.amazon.com/Kensington-Orbit-Trackball-Mouse-Scroll/dp/B002OOWB3O

The only thing I don't like about mine is that it doesn't have a middle button...

I have two of those, try to chord and move the ball or scroll ring at the same time.  Also, so light they don't stay put on the desk...

I tried both buttons and the ball and it didn't work; didn't think of the scroll ring; I'll have to try that  next time I boot up my Poser machine. I never had any problems with the weight, because the rubber palm rest holds it down.  It's much better than the Logitech Marble it replaced. However,  I still miss my old Microsoft mechanical IntelliMouse Trackball...

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The Wisdom of bagginsbill:

"Oh - the manual says that? I have never read the manual - this must be why."
“I could buy better software, but then I'd have to be an artist and what's the point of that?"
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