I've been standing all day for three years. It takes a few months to get your legs used to it. Our office has 20 of these:

http://www.ergodepot.com/Ergo_Depot_adjustable_desk_AD127HD_...

There are a few exercise balls and aerons around that people occasionally use. I have a footstool that I rest one leg on as I lean forward on the desk. The other footstool is for pairing with a developer who is much shorter:

http://pimen.to/office-desk.JPG