You only need 3 things to build a gps tracker: gps module A1035D, mobile phone Motorola C168i, microcontroller ATTINY84-20PU... and just a bit of patience. This is a complete project and localization is performed by inserting latitude and longitude in Google Maps.

It is very useful for developers since firmware for Atmel AtTiny is available and easily adaptable for whichever mobile phone or gsm/gps module.
Without doubt, this is an excellent open source GPS tracker, more over there is a forum for users to deepen their understanding of project details.