Make:

Now this looks like fun:

The Open GPS Tracker is a small device which plugs into a $20 prepaid mobile phone to make a GPS tracker. The Tracker responds to text message commands, detects motion, and sends you its exact position, ready for Google Maps or your mapping software. The Tracker firmware is open source and user-customizable.

From the looks of things, the total cost to build a remote-operated GPS tracking unit is on the order of $100. The design uses a prepaid cell phone to receive commands and report its position via SMS.

I'm sure there are a number of boring nefarious application for this that will freak out a lot of folks, but just think about the more interesting possibilities. You could add this to a weather balloon or autonomous flying vehicle easily track it down if there were any flight problems. A bunch of people in any city could put these in their cars on a short time delay and automatically report traffic flow conditions. You could even roll your own "lo-jack" system that would let you find your car if it was stolen, only with this your car's location is only being reported to you instead of a monitoring station, actually increasing your privacy.

Open GPS Tracker