The best ambidextrous mouse I've seen it the
Logitech MX310 I mentioned further above. It "only" has 6 buttons, and the one under the scroll wheel is kind of hard to use, but it sure is nice otherwise...
Edit: For awhile when Jennifer and I were sharing my computer after we got our house we just had two mice, one on each side of the keyboard...
Edit 2: For the longest time I used to remap movement over to the arrow keys for use with my left hand! 
I'm not quite sure why... However, I think it was when NOLF came out that I finally went with WASD because I needed more keys nearby for all the different functions...
BTW, does anyone out there use the right mouse button to jump? None of my friends did when they gamed and they all thought I was crazy, but I swear by it.