Prey is a free and open source laptop tracking software developed by Fork, that helps recover computers by tracing their location through the remote retrieval of information. The software is activated from the outside by using a URL as a checking method, and phones home by sending a detailed report either to a centralized control panel or directly to the computer owner's mailbox.
Basically you install a tiny agent in your PC or phone, which silently waits for a remote signal to wake up and work its magic.
This signal is sent either from the Internet or through an SMS message, and allows you to gather information regarding the device's location, hardware and network status, and optionally trigger specific actions on it.
http://preyproject.com/
