Mike writes in about his super-cool Open GPS Tracker.
Here is the simplest build-it-yourself GPS tracker yet. No expensive GSM module required – I am using the $20 GoPhone from WalMart or Target. Uses a 14-pin AVR microcontroller, a voltage regulator, a transistor, a GPS module 2 inches across, and that’s it. My open source firmware can track objects automatically or manually. Has a programmable command interpreter that can sense or control pretty much anything by text messaging.