Inspired by the commercially available and all-too-pricey balance ball desk chairs out there, I decided to make my own with the ball I already I own, some PVC, wheels and a few tools. The ball can also be very easily removed (simply lifted out), so you can use it for working out. This is an inexpensive and easy project that will (hopefully) force an end to slouching and work those abs while 'working' or in my case 'studying' (yeah, not so much studying as fantasizing about bookshelves and LED throwies while on Instructables).
Note: If you have back/neck problems or at all concerned about how this might affect your back/neck/whatever, please consult your doctor or physical therapist. I am not responsible if injury or further injury occurs to you or anyone else as a result of this DIY guide.
Step 1: Supplies
Balance ball (I used a small sized one, but I think a medium might be better; the large might be too high for the desk)
4 12" pieces of 1 1/4" PVC pipe
4 1 1/4" PVC elbows
PVC primer & cement (I have ones with the brushes inside; if you don't have that, add brushes to your supply list)
4 2" twin wheel casters with stems ... the ones that have the metal piece that the stems snap into (brakes are a nice feature too)
Spray paint (or duct tape, which is what I had on hand and used ... but probably does not look as good)
Saw (if you need to cut the PVC down)
Sandpaper (I had 150 grit, which worked fine)
Drill & 3/8" bit (to fit the wheel stem, so adjust bit size as needed)