I finally jumped on the Arduino bandwagon in April of 2010 when I got a Duemilanove board as a late birthday present. Later that summer I got a GPS shield and GPS receiver to go with it, and even later that summer I got an LCD display, Blackberry rollerball board, and microSD shield to round out the package. And with that, I had enough to build a carputer. It does two big things right now: displaying bits of info to me while I'm driving, and recording everywhere my car goes for other purposes.
I started out with just the GPS shield, which made for a pretty limited carputer - unless it was hooked up to a laptop, any data it collected disappeared immediately. After I collected the SD shield and an LCD display, things actually started shaping up. Here's a bench build of what is very close to the final configuration.
I needed a good place to put the carputer in my car, and it turns out the trunk was perfect. I had an extra unused power outlet back there, plus there is a little enclave at the top that I was able to stick the Ardunio board in. After watching my car's output voltage with a multimeter, I decided to build a simple voltage regulator to drop the voltage down to 9 volts and keep it pretty constant. It wasn't anything sophisticated, so it combined with the Arduino's built-in regulator seem to work well. I just used velcro cable ties to hold everything to the sides of the trunk.
Here's the user-facing end of the carputer, an LCD display and rollerball mounted to the dashboard. The black cable sticking out the side is a CAT5 cable that runs to the Arduino board in the trunk, carrying power (+5V and ground) and signals (roller up, roller down, roller click, LCD TX) for the rollerball and LCD display. This picture shows the main screen, with time, speed, altitude, and heading displayed.
Scrolling the rollerball down moves to the next screen, which shows latitude and longitude of the current location. It turns out this info is really only good for trivia. Who can make use of lat/lon in their heads?
The final display screen shows the current distance on the trip odometer. This odometer is reset each time the carputer is powered up, but can also be reset via a menu option. The main menu (not pictured) is accessible by clicking the rollerball, giving me access to timezone settings, display modes, software version information, and a few other things.