The Hamster Wheel
NOTE: User was awarded 25 shmuckers for this post. -Rob)
I purchased a cheap cycle desk from Costco a few months ago. Adjustable height / tilt on the desk area, adjustable seat height. Very effective, and it doesn't feel like a chore to hop on and do a few minutes at a time.
Those little crappy pedal things that you're supposed to use with a normal chair are awful, though. Extremely uncomfortable and they don't do much. You can't even use those with normal desks, because your legs will be hitting the table.