If you live in the United States or Canada, there are a number of different sites that help you find cheap gas stations. Some of the best sites are Gas Buddy, Gas Price Watch, and Fuel Me Up.
Each of these sites are maintained by the users. If you see a better deal on gas in your area, you can add the gas station and it will show on the site.
In order to make sure you are getting the best possible price, check for cheap gas stations in the areas all around your daily commute. If you can drive one mile off the beaten path to a gas station with a .10 discount or more per gallon, that will make a significant difference in your wallet!