GPS or highway navigation computer. I use a Garmin Nuvi on the road, but I use it only to know where I am, my travel plan is worked outbahead of time, in my head, and flexible. So the cheapest Nuvi works fine for me.
But my GPSMap 60c (since superceded) is far superior as a GPS since it will work with as many birds as it can find on 12 channels to statistically give me a better estimate of location. Almost any recreational GPS receiver is a better GPS than any automotive navigation device, it is the difference between putting you someplace on the nearest road (represented as several hundred feet wide) or telling you that in two more steps you will fall off a cliff.