If you have an outdoor pet and are curious what they get up to during the day, this TinyDuino-based collar may be just the right project for you. It doesn't let you track location remotely, but it does log GPS coordinates so you can do a little pet-snooping.
Ken Burns over at Make has put together another fun-looking project. While you can buy products that let you track your pet remotely via GPS, that's not really what this build is all about. Instead, it logs GPS coordinates to a microSD card. You can download the data to a computer at the end of the day and view it in a tool like Google Earth, complete with time stamps. So aside from being an interesting project, it's a fun way to actually see your pet's daily wanderings.
To put it together, you'll need a TinyDuino microcontroller (much like the Arduino Uno, but smaller), a GPS TinyShield, and a microSD TinyShield. Check out the full build instructions at Make.
GPS Cat Tracker [Make]