Fleet Online lets you find the location of a mobile phone. Once the target phone has agreed (via a confirmation sms) to being tracked, its location can be requested for a cost of around 25p. It’s no where as near as accurate as GPS, but you don’t need any special equipment. Precision depends on location (built up areas are more accurate) but it tends to be correct to within a couple of miles. Lots of privacy concerns, but it’s going to be big.

Not quite what you expect to see through your window while you’re having your tea.