Basically, it's a stationary bike with a laptop stand. It features a belt-driven pedal system with adjustable resistance, a folding design so you can stow it in a corner of your office, and a rather goofy-looking fabric-draped foam shelf for your laptop. (There's also space to rest your forearms, which I think is a huge plus.)
Here's a quick video that shows the FitDesk in action:
The idea, of course, is to spend, say, 20 or 30 minutes per day pedaling while you're doing routine work stuff: answering e-mail, updating Twitter, and so on. And it's a fairly brilliant idea, if you ask me, because it enables you to kill two birds with one stone.
The FitDesk normally sells for $289.99, but right now it's on sale for $199.95. Of course, if you already own an exercise bike (or treadmill, elliptical, or whatever), you can add your own laptop stand on the cheap. Or, if you're not the DIY type, check out the $39.95 SurfShelf portable treadmill desk.