Inspired by ElevatorHacking. It is not legal to modify your car's computer but there is an interface called OBD-II (like a big serial port usually under the back of the steering wheel) that allows you to receive (and in some cases send) all kinds of data from your vehicle's engine. It was mandated for all cars after 1996 primarily to monitor emmissions but is used by techinicans to do engine diagnostics. Computer and HandHeld software is available to allow you to see this readily.
Used by companies such as OnStar? in conjunction with wireless for instance to remotely unlock your car if you lock your keys inside.
There have been rumours of GP motorcycle teams reproing race bikes on the grid to evade certain regulations only checked in the paddock and so improve their chances of winning.
See IfYourCarWereEmacs, MyMotorcycleIsEmacs