The idea is to find the lowest-priced fuel in your area. As a resource of great potential, Mapquest Gas Prices has built in a filter to find all sorts of alternative fuels: biodiesel, E85, hydrogen, EV charging stations, CNG and more. If you don't have an alternative-fueled car, you can also find the lowest price for standard gas in your area. You need to at least select a fuel type and a city and state (or zip code) to get results. You'll have to do the math yourself using your car's mpg to find out if it's cheaper to drive there vs. paying a few more cents per gallon around the corner (the environment would appreciate it if you visit the local store instead of driving somewhere else to fill up, but if the cheapo gas is on your way somewhere you're already going, then by all means...). You can do this using the pop-up gas price calculator, which is actually kind of fun to play with. Here's one example: an 80-mile-trip in a gas guzzling SUV (at 15 mpg) with gas that costs $3.08 will run you $16.43 total. That same drive in a 40-mpg hybrid will be just $6.16. You can input trips up to 9,999 miles, so some people will be able to use this to calculate average annual fuel costs for your car if you don't want to do it by hand.
The page just launched today, and there might be a bug or two. I can't get it to load correctly in Firefox, but it looks just fine in Safari. Also, as a disclaimer, both Mapquest and AutoblogGreen are AOL properties.