Studies have shown that when you're working at your computer, you should take the opportunity to stand as much as possible. It's better for your health. And the office furniture market has responded, with an explosion of standing desk offerings.
But sometimes you need to sit, so it's good to be able to switch between sitting and standing with your computer. Ergotron's WorkFit uses an arm to that can raise your Mac when you're standing, and lower the computer when you're sitting.
Macworld actually reviewed the WorkFit-A when it was first released. Designed for the iMac, the build quality and features of the WorkFit-A are great, but at the time of the review, the monitor would jiggle when typing. An Ergotron representative told us that this issue would be addressed with a special adapter that helps steady the WorkFit-A.
The WorkFit-A isn't the only standing/sitting desk by Ergotron. They have a wide variety of desks suitable for any work area.