Ergotron Workfit — Sit-stand adjustable arms, mounts and desk conversions.

These Workfit models have been around for a while but don't get mentioned much, I really like them as they swivel and effectively disengage you from a fixed orientation, plus are so easy to push up or down—there are no fixed heights. In essence this enables you to not move your chair when standing up or sitting down, instead you just step to the side. They are fairly expensive, although compared to the cost of some of these pieces of veneered board with adjustable legs that are being punted, the engineering you get with these is seriously heavy duty (weighs a ton too). There's a newer 'platinum' model for iMac users that looks swish. Most useful is the model with a primary monitor mount, laptop stand (as second monitor), plus keyboard and trackpad tray (bluetooth). Depending on how far you're willing to push your laptop to the side you can get a monitor of up to about 26" but 23" is better beside a 15" laptop. They aren't really a desk—but a workstation. Can use the arm connecting the keyboard tray to prop a coffee and some doodads though ;) You can attach them to your desk or rather better is to attach them to a wall, shelf or pillar (requires a bit of custom mounting—get the heigh right!) then say goodbye to clutter. Be warned that they have to be mounted perfectly flat (else the swivel is useless) and if mounted on a desk, get rid of the rubber feet that raise the keyboard platform too high. The biggest disadvantage is that you get a bit of a wobble if you hit your keys too hard, however I've gotten use to it and tap away lightly. And no it doesn't descend unless you push a bit harder (there's no position lock).