The AAA has improved their TripTik online mapping tool to include gas prices and locations for about 100,000 places in the US. You can try TripTik free (you don’t have to be a member) at AAA.com .
You’ll notice that it looks much like other mapping applications that you’ve used, except you get more options for point-to-point travel (you can travel between sports venues, for example, or hospitals, or cities, instead of being restricted to address-to-address mapping.)
I decided I wanted directions from Hollywood Florida to Hollywood California. After a cute little animation I got directions for the 2700+ mile trip (40 hours!) There are several tabs; the first one you see is for directions. The Travel Tab provides the gas information; it’s the last icon on that tab. When you click on it at first you won’t get any additional information since you have to zoom in pretty close (like three notches from the closest zoom) to get the gas information.
When you see a Gas icon, just mouseover it for information. You’ll get the name, address, and phone of the gas station. For some stations you’ll get no gas prices at all, for some you’ll get only a single price (just diesel, just regular, etc) and for some you’ll get several prices. In addition to the price you’ll also get a note of when that price was recorded. From what I could tell the prices are updated fairly frequently, though in my experience gas prices can change multiple times a day.
You can add locations to your trip or just click on a balloon to keep it “pinned” to the display, making it easy to compare multiple locations. I didn’t like the fact that I had to zoom SO FAR IN to get the prices (made it harder to keep track of the trip as a whole) but I liked the information here and the update frequency.