You’ll be able to find out where the best rest stops are, get maps and updates on road conditions, and get a sense of where you may encounter speed traps. ViaMichelin also calculates the price the kilometers will cost you at the gas pump. (It will help to select a rental car that will go far on a liter of gas.)
Those of you stateside can use the Low Gas Price Google widget to find low prices in your area, Google Maps to get traffic and street level views of various cities, and Trippish to get weather reports on along your course.