MMM recently recommended GasBuddy for finding the cheapest gas in the area. However, I haven't seen anyone here mention their fuel logbook. From the main site,
gasbuddy.com, you have to select one of the local sites and create an account there. The "Fuel Logbook" is in the middle of the menu at the top of the page. After you add your vehicle(s), just start tracking your fuel purchases. You must FILL UP for this to work.
This is how I do it:
Fill up the tank until it clicks once
Get a receipt
Write the current odometer reading at the time of fillup
Write down the price per gallon and number of gallons if it is not on the receipt
Enter the info into the fuel logbook
It keeps track of your expenses for each individual car by calendar year.
For those living outside the US, you can just put in your local currency and number of liters in the gallon spot. I am currently doing this since I'm in Japan.