I built a standing desk out of 2 x 4s and my old desktop, a piece of cheap particle board I've been hauling around for a decade or so. I already had an articulated keyboard tray I'd salvaged from a previous employer, so I put that on it for fine adjustments.
I built a so-called "tie-fighter" keyboard out of an old Goldtouch. http://www.instructables.com/id/How-to-make-a-vertical,-ergonomic-tie-fighter-ke/
I also built a raised mouse platform to go next to it and screwed it to the keyboard tray, so that my arms would remain at the same height for typing and mousing.
I got an inexpensive ergo mouse. http://www.amazon.com/Vertical-Ergonomic-Optical-Mouse-Ergoguys/dp/B001FWKA7A/
I got an anti-fatigue standing mat, the kind which are used by cashiers and such. http://www.amazon.com/Crown-Comfort-Antifatigue-Zedlan-CK0023BL/dp/B000PTO8MW/
Probably the most expensive thing I did was buy a new, drafting-height chair. See other comments in this thread about bar stools; same idea.
The end result is an ergonomic workstation that is almost perfectly suited to my ergonomic needs. I no longer pronate my wrists; the only tendon that gets tensed is the one around the back of my elbow. My back doesn't hurt from sitting anymore and I have better circulation overall. I would say I sit down about 25% of the time when computing, though it varies from day to day. Some days I won't sit hardly at all, others I'll feel a bit lazy and sit maybe half the time.
I chose not to spend lots of money on a commercial standing desk or one of those movable ones. It seemed easy and approachable to do some simple carpentry and build one myself. The whole project increased my awareness of computing ergonomics, and I think my body is happier for it.