It's the nightmare nobody wants to think about: somebody's taken your laptop. Sure, maybe it was just an accident, since many of them look the same. but nevertheless, whether through careless mistake or by deliberate theft by some low-life, you are here and your computer is somewhere else. With the cost of your system—to say nothing of all the important data that lives on it—on the line, you'd sure like to get it back, pronto.
Prey is a tool that just might give you a hand with that. Obviously you have to install it before your laptop goes missing—something about closing the barn door after the horse has already escaped—but it can then assist you in the recovery effort. The way it works is this: at a pre-determined interval, Prey checks for the existence of a specific URL. If it finds it, everything is just hunky-dory. But if the machine has gone missing, all you have to do is to get rid of the page at that address, and Prey will know that something's up. At that point, it will do things like report on where it is and what it's doing—and if you've got a webcam, you may even get a picture of the thief. It can use a wired connection, or search for a viable Wi-Fi signal, and for Mac and Linux systems, you can install the app as Root, so your thief doesn't even have to be logged in to let you know what's up.
Prey is available for Windows, Mac, and various Linux distros, or if you're really feeling adventurous, you can grab the source code and roll it yourself.