There are lots of companies selling adjustable-height desks, but they tend to cost nearly a thousand dollars, or more.
I finally found something that works and doesn't cost so much: http://heightadjustableworktable.com.
If you already have a desk, you have a desktop, and can probably reuse it, so all you need is the frame. (If you don't have a top, they sell complete desks too.)
The frame breaks down into a fairly small, dense package.
It only took about an hour to assemble (not counting idiotic rework because a certain hamfisted idiot flipped the top the wrong way the first time).
The mechanism is metal on nylon bushings, and operates smoothly.
The engineering and manufacturing are first-rate.
A very good value.
My only complaint is that the lowest height of the desk is still too high for a normal-height woman, and there's no way to shorten the legs because the mechanism is integral.