There are some types of height adjustable desk which don't fall into a particular category. The first of these isn’t really a desk at all, instead it enables you to make you fixed height desk into an adjustable height one. How is this possible?
Desktop height adjustable computer workstation
It’s actually quite simple because what you have is a stand-alone unit designed to sit on top of your existing fixed desk. It has its own worktop where you place your keyboard and mouse. At the rear is a column with a plate to attach your flat panel monitor onto. Once it’s set up you can move its worktop and your flat panel monitor to the height you want to work at.
This is quite a neat and cost effective solution to sit stand working, however it does have limitations on weight carrying capacity and the worktop size is only big enough for keyboard and mouse.
Height adjustable desks for laptops
As laptops are light and easily transportable they have opened up a new style of height adjustable desk specifically targeted at the portable computing market.
Because laptops and notebooks are both lightweight and compact a lot of the robust engineering needed for standard desk height adjustability isn't necessary. As a result it’s relatively easy to produce sit stand units specifically for use with this type of computer.
Generally the units use either crank or gas operated height adjustments as well as telescopic tubes where a side-mounted knob can be used to lock and release the height positioning.
As long as you don't need a lot of additional workspace these units can be quite effective. They're also often made with casters like you see on office chairs and can be quickly and easily moved in and out of a work area. They are popular for businesses for hot desk use where occasional visitors to the office can be set up with a compact workstation at very short notice.
Herman Miller Envelop Desk
Herman Miller’s Envelop desk adjusts by means of re-locatable pins covering a relatively short height range. It’s intended for sitting use rather than standing and sitting.
However, because it allows you to move the top in towards your body it actually helps you to sit in a lot more open and healthy posture. You can find a review of the Envelop desk here.