Open GPS Tracker
"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.
Project status: Current build is 0.17A assembled 05/09/2008 We currently have second-generation stable firmware and a reference hardware design. This combination has been extensively tested in moving vehicles and found to be reliable. There have been no hangs since build 0.14, and no lost messages since build 0.17 was released.
All parts are available from Mouser Electronics, and the phone is available from Target, Walmart, or Radio Shack. This site provides the firmware with source code, theory of operation, parts list, and exact assembly and checkout instructions. If you can solder, this is a one-sitting project. No PC board or surface-mount capability is required." (http://www.opengpstracker.org/)