I'm probably about to impart too much information, but my tail bone was broken eight years ago. A tail bone is supposed to have a natural curve, but mine healed straight, which makes sitting for any length of time uncomfortable. With all the time I spend at the computer, I sometimes go through periods of time where I've crippled myself with all the sitting. Yeah, it's a lot of fun.
But behold! The Gaiam balance ball chair! Josi Kilpack has one of these in her office, and I had the chance to try it out. Upon sitting, I knew immediately I had to have one, and I ordered it from Amazon. It arrived a week ago and it has made all the difference in my sitting experience.
Why is it so wonderful? First, you can adjust it to your own firmness needs. If you need a squishier chair today than you needed yesterday, you just let out a little air, or pump it up a little, if that sounds better. Second, it arranges your body ergonomically, so your spine, thighs, and knees are all where they are supposed to be. Third, the back rest is in just the right place.
So if you're having "sitting-at-the-computer-and-my-back-and-behind-hurt" blues, this just may be a solution for you. I know I'll never be without mine.