RoadTrippers – an excellent resource for discovering, or planning, road trips. You can select if you want to use highways or back roads to get between stops, as well as estimate fuel cost and time of driving.
RoadTrip USA – this book ignited my passion for making a cross-country road trip. It’s the first resource I picked up, and I took pages upon pages of notes.
Atlas Obscura – a great way to find off-the-beaten-path adventures.
More will be added!