Prey helps you find your stolen laptop by sending timed reports to your email 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.
Prey can use a web URL to check if it should generate and send the report, so you have a way of alerting remotely the program whenever your laptop disappears. It can (and should) be run as root so it doesn’t depend on an active user session to run, but only on a succesful boot.
http://preyproject.com/
[Moderator note: although a great program, this is clearly not portable. Moved out of "Submission" forum.]