Linux

While I never had one I remember following the build page, and I was crushed when the price point was far too high for me to even consider getting one. However I did ride around with a carputer for some time before an affordable commercial head unit with CD/MP3 capability became available.

It was an off the shelf IBM desktop in the backseat, powered by an inverter, and controlled by a numeric keypad stuck to the center console with some velcro. Getting music on it required hauling the whole thing in the house, but at the time it felt pretty awesome to have pieced together a solution that worked.