Live Track Any UK GSM Cellphone

World Tracker turns anyone into a cellphone spy - Engadget

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.