Interfacing a Arduino to your car or truck computer allows you to monitor the performance of your vehicle and watch for and explain trouble codes. You could build your own custom gauge display, monitor fuel consumption, turbo boost pressures and more. There's a bit of interfacing to do, but there is a chip that talks most of the different protocols in use by the various manufacturers. A good start on the process is explained at http://code.google.com/p/opengauge/wiki/OBDuinoInterface, and details on the interface chip can be found at http://www.elmelectronics.com/DSheets/ELM327DS.pdf. For more details on OBD-II, see http://www.obdii.com/.