A UK service called World Tracker apparently uses cell tower data (or GPS, when available) to track the location of just about any GSM cellphone. Just enter the number you want to track into the service's handy Google Maps-based interface, and you'll be able to zoom in on the device's location, with accuracy somewhere between 50 and 500 meters. The first time you try to track a phone, a text message is sent to the owner, who must reply in order to enable tracking. The service is currently compatible with O2, Vodafone, Orange and T-Mobile in the UK, and has plans to expand to other markets including Germany, Spain, Norway and the US.