The Garmin ecoRoute HD Vehicle Diagnostics Communicator is a cable which plugs between some of the Garmin nüvi GPS units and a car's On-Board Diagnostics (OBD) interface, turning the GPS screen into a car diagnostic system.
The goLINK OBD-II is a similar cable to interface for an Apple iPhone or iTouch and the OBDLink Bluetooth interfaces via Bluetooth. You can also build the blueOBD Kit and interface it to an Android phone.
If you want a dedicated device there is the Auterra DashDyno which has its own screen and can be fitted into the car dash board. Linear Logic make a cheaper dash mounted unit, but without GPS.
If all you want to do is actually work out what is wrong with your car, rather than show off with a flashy display, there are simple low cost calculator like OBD-II Code Readers.