Prey can be configured to "phone home" every time the laptop is turned on, or only if it finds a URL that you specify during setup. The idea of the URL approach is to minimize spamming yourself; if the unthinkable happens, just quickly configure the URL (such as a subfolder at your blog site, for example). The next time the laptop is turned on, it transmits a bevy of data, including network information, what programs are running, and even a snapshot (if the system has a Webcam) of the perp.
The bad news: Prey hasn't been translated into English yet. Not the Windows version, at any rate -- Mac and Linux users are good to go. That said, I had ho trouble installing and configuring the program anyway, and I don't even speak Pig Latin.
Prey isn't the only program of its kind, but it is free and easily configured. If you're looking for other options, don't forget to check some other freebies, including LocateMyLaptop and Loki, plus the $50/year Laptop Cop. [via ghacks]