The website GasBuddy.com allows you to search for the cheapest gas prices by ZIP code or by city. Another good source for gas price information by ZIP code is Mapquest.com.
Both of these websites allow you to choose the fuel type you are interested in, and they also provide detailed location information for gas stations. The gas price information available at Mapquest.com may be especially useful for trip planning, as you can search for gas stations close to your route and rank them by either price or distance. GasBuddy.com also provides a calculator for estimating your trip costs based on gas prices.