I've been sitting on an exercise ball for the last couple years, since I read a local newspaper article about schools in my community that offer those to schoolchildren to sit on. But only last week I discovered that most school exercise balls have knobs on them that keep them in place. Mine is a regular ball, which can roll in any direction. Maintaining my balance provides subtle movement any time I am at the computer, which is a lot of each day for me.

P.S. What do all of you recommend for more ergonomic keyboards and pointing devices? I am not happy with my Dell keyboard nor with any mouse, but the last trackball I tried was even more annoying than my current mouse. (I have liked trackballs very well in the past, but I have to favor my thumb basal joints now that I am older.)