I like Roadtrippers a lot. During a trip I use it to find POIs along the route-- especially things like museums, abandoned stuff, etc. that might not pop up on Google Maps without specifically searching for it.
It's also great for route planning, since you can add your waypoints and it will calculate mileage, drive time, and even fuel cost based on your car's gas mileage and current prices. Then you can toggle the waypoints on and off to see the impact. You can also just add a bunch of points to the map and it will calculate the most efficient route to hit them all. On a long trip I also use the estimated drive hours calculation to estimate whether I'm on pace or if I need to modify the route. I'm sure other apps do this too, but the UI is really easy to use.