Gas Buddy and Fuel Cost Calculator
These are two really good site for road trips. The first is also an app, I believe, and it tells people where they can find the cheapest gas station in the area they're in. Fuel Cost Caluculator lets you estimate how much your road trip will cost. You type in the make and year of your car and where you're going and it tells you how much you'll likely spend on gas. Its run by AAA, I'm pretty sure.
I used both on a recent road trip to Florida and I thought they were really great.