
The handy Butterfly GPS is a cost-effective system that presents location, date, and time data on its LCD. The system features an ATmega169 and a highly sensitive GPS receiver engine board based on the SiRF chipset. Its amazing what you can do with such a low cost development board.
AVR Butterfly GPS System: [Download Project] - [View Project PDF] - [Via]

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. This looks like a great project if you’re into GPS projects.
Open GPS Tracker System: [Link]

This GPS based car speed and position logger is based on Olimex LPC2148 project board, which is wired to GPS receiver. GPS receiver is interfaced via serial port - so not much of soldering. Software is based on FreeRTOS where ChanFS file system is used for storing logged data in SD/MMC FAT16 formatted card. It also has USB serial port emulation which enables to monitor NMEA sequences or simply retrieve logs as from mass storage device without removing CD/MMC card.
LPC2148 controlled GPS car speed and position logger: [Via] - [Link]

There are already mobile tracking devices out there, but they seemed to be too expensive and too closed for various needs. Another option is one of these new Nokia N95 which have built-in GPS. They are really nice, but about 600€, which is not a bargain. So it might be a good idea to build your own.
Interfacing an AVR controller to a GPS Mobile Phone: [Link]