In our experience there's a lot of interest in GPS trackers, like the BlackLine GPS Snitch we reviewed last year, but most people can't justify being locked into long-term, often expensive service plans. That's where the Open GPS Tracker project steps in: it aims to take cheap, prepaid cellphones and fit a simple GPS adapter for a low-cost alternative.
The phone of choice right now is the Motorola C168i AT&T GoPhone, which you can find for around $20 at convenience and drug stores. Into that plugs a straightforward breadboard with an Atmel ATTINY84-20PU AVR microcontroller, Tyco Electronics A1035D GPS module and a few other components, together with a battery supply.
Like the commercial units, the Open GPS Tracker can respond to movement or an SMS message, and sends out the exact GPS position so you can display it on Google Maps. If it's out of coverage then it caches the locations and serves them up once it finds another signal.
If you're handy with a soldering iron, and fancy saving some money, this could be a really useful project to attempt.