Closest I've ever come is using a workbench or a drafting table for extended periods of time.
In the case of the workbench, I felt better at the end of the day than I normally would if I'd been sitting. The drafting table was more of a mixed experience. Maybe it felt a little better than sitting. But still not much, since I tended to lean whenever I was writing. So I think feeling better had more to do with all my moving around while at the workbench than it did with my not sitting at it.
Tried the exercise ball thing. Didn't like it. I couldn't get decent enough posture while trying to keep it where I wanted it. I also tend to roll slightly forward and back depending upon what I'm doing. (Closer in for most typing, further back for predominantly mousing tasks.) After a while, my lower back started getting sore with the ball. (Somebody told me it worked better if you sorta bounced slightly while you were on it?) Also bought one of those "kneeling chairs" that used to be in vogue. I liked it. My back felt great. But my knees started aching after a few days. Stopped using the chair and the pain went away. Started using it again and the pain returned. In the end, I gave it to a friend and never looked back. Been using a good ergo-type chair ever since. No regrets on that score even though I'm painfully aware I sit in it far too many hours straight for my own good.
I'm gonna side with f0dder. I think he's right when he said regularly changing positions seems to do more than anything else.
And I'll also be the first to agree that just sitting all day is probably the worst thing you can do.
Hmm...maybe I should try a standing desk? They're easy enough to build...