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...

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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?"
"The [R'osity Forum Search] 'Default' label should actually say 'Don't Find What I'm Looking For'".
bagginsbill's Free Stuff... https://web.archive.org/web/20201010171535/https://sites.google.com/site/bagginsbill/Home