Best Buy Sells Surveillance Tracker
Keep tabs on your child at all times with this small but sophisticated device that combines GPS and cellular technology to provide you with real-time location updates. The small and lightweight Little Buddy transmitter fits easily into a backpack, lunchbox or other receptacle, making it easy for your child to carry so you can check his or her location at any time using a smartphone or computer. Customizable safety checks allow you to establish specific times and locations where your child is supposed to be — for example, in school — causing the device to alert you with a text message if your child leaves the designated area during that time. Additional real-time alerts let you know when the device’s battery is running low so you can take steps to ensure your monitoring isn’t interrupted.
Presumably it can also be used to track people who aren’t your kids.
EDITED TO ADD (11/12): You can also use an iPhone as a tracking device.