Here’s a great post from a medical student on exercise balls. I’ve added some links at the end of the post if you are interested in learning more. 
9 Of The Best Stability Ball Exercises You’re Probably Not Doing
First of all, I wanted to tell you how much I’ve enjoyed your blog. I started reading it about 7 months ago when a friend suggested it. I especially appreciate the recipes and notes on working out (great running post !!!!).
While it’s not a major topic, I did want to suggest you might do a post about exercise balls. I’m going to rave a bit, but that’s the gist of this comment. I got one last year when I noticed that while I was able to stay aerobically fit with an efficient running routine, I was too busy to do consistent weight-lifting and ab exercise.
The exercise ball has been AMAZING – I use it as a study chair to keep myself awake if I’m especially restless or sleepy (it works!) and I take 3-5 minute “ab breaks” fairly often when I’m studying to work my abs and refocus. The results have far exceeded my expectations. It’s more effective (for me) because it adds consistency to ab workouts, which I’ve found especially critical to seeing any improvement at all. My back also tends to stiffen up a lot when I study for hours on end and switching to sitting on the ball alleviates that tension/stiffness, as it forces me to engage “core” muscles.
There are countless free online tutorials for different exercise ball workouts (abs, arms, back, etc) for those interested in getting creative.
Given your post on push ups, I might add that I use the ball for push-ups as well to get emphasis on those core stabilization muscles. Also, these are CHEAP (less than $20 from plenty of sites found through Amazon).
Katy Bowman
Choosing and Using an Exercise Ball from about.com