Prey lets you keep track of your laptop, phone and tablet whenever stolen or missing -- easily and all in one place. It's lightweight, open source software that gives you full and remote control, 24/7.

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

(Source: http://twitter.com/inkedmn/status/205715554059825152)