Standing desk

I have a treadmill desk, I love it. I bought a used treadmill off Craigslist. I got a piece of thick foam for about $2 at Home Depot, along with a piece of shelving ($10). The only requirement going this route is that the arms on the treadmill have to be completely straight to hold some kind of shelf.

Obviously building one that isn't situated around a treadmill would be different, but I do stand on mine when I don't want to walk, so I like that I can do both.

i like the idea of a treadmill desk, but i feel like i'd be REALLY non-productive at it.  how do you read/type/do work?!?!?

You walk at 1-2.5mph. You're not running. It was actually quite a natural transition and reading and typing aren't an issue. I walk about 3mph when just watching tv. When I walk for over an hour and get a little tired, I just turn it off and stand.

I have one at work but it's simply a raised work surface in my cubicle. 
The office chair, if I want to sit, is bar-stool height.

My boyfriend works from home. He and his employee got these standing "wellness mats" which they said helped because your feet can feel really tired by the end of the day:

http://www.williams-sonoma.com/products/wellness-mat/