I know several riders who use a FitBall or something similar to improve their balance during the winter when they can't ride. I've done it in the past and was pleased with the results.
This year, Gymnastics Boy (my personal trainer), has had me doing a ton of work on a Bosu. We started out with it platform down, so that I was standing on the squishy-ish blue ball portion. He had me doing squats and basic balance work to keep my core working. He then flipped it upside down, like this:
I have to tell you, while I was impressed with the FitBall's help with my balance, the Bosu has it beat hands-down. I've never felt more secure riding. I realized it on Sunday when Estes was wound up - it was a breeze to stay with her. My confidence in my balance has skyrocketed, which directly translates to my confidence on Estes.
I even felt comfortable enough to really get after her about halfway through our ride. Usually, I'm a little hesitant to really correct her when I'm riding bareback. She was being a bit of a mare and I corrected her harshly, which involved some quick movements during our discussion. It wasn't until I had my nice mare back that I realized that I'd done it bareback and without a second thought.
I can't recommend the Bosu strongly enough for all riders - not just the ones who feel like they need help with balance. Heck, when I started working with the Bosu, I was a bit cocky because I *knew* that I had good balace from riding. Boy, was I wrong.