From PreyProject.com

Prey is a lightweight application that will help you track and find your laptop if it ever gets stolen. It works in all operating systems and not only is it Open Source but also completely free.

Recover what is rightfully yours

Prey helps you locate your missing laptop by sending timed reports with a bunch of information of its whereabouts. This includes the general status of the computer, a list of running programs and active connections, fully-detailed network and wifi information, a screenshot of the running desktop and — in case your laptop has an integrated webcam — a picture of the thief.

Silent but deadly

Prey uses a remote activation system which means the program sits silently in your computer until you actually want it to run. If so, it gathers all the information and sends it to your Prey web control panel or directly to your mailbox. The thief will never know his movements are being watched.

And because there’s always hope

You may be thinking “but what’s the point of this program if the guy will probably just format the thing right away?” and you’re completely right. However, experience shows that thieves tend to look in stolen computers for valuable information, so there’s actually a chance you can catch the guy (and there’s even some successful cases!).

Besides, if by using Prey you can keep alive a tiny bit of hope that you’ll recover your computer, isn’t it already worth it?