The recently released CATS.i measures just 45mm x 35mm x 12mm, and makers say it can be used to track cars, pets and even people. Given that they also claim this is the "most covert" tracking device of its type and is "almost undetectable when being worn", I'm not sure I want to know who their normal customers are.
The device uses a built-in GSM sim chip to report back its GPS data, either constantly or at set intervals, which can then be tracked online. It is battery powered but can also be hardwired to vehicles, or even solar power—though the makers don't specify how. Quad band GSM, GSP and RF ensures you should be able to monitor the people you are stalking—sorry, tracking—wherever they go. [Cats.i]