"Gas" is an amazing Dashboard widget that shows you where you can currently get the cheapest gas in your area.
Whether you re going cross town or cross country, Gas delivers information from gaspricewatch.com in order to provide you with the most up-to-date gas prices available. As gas prices are on the rise, this widget rises to the challenge of saving you those extra cents as you go about your journeys. Have a nice drive.
I've been using this for a little while now and it's great. As someone who didn't used think too much before I got my gas, it's been a real help. And it's saved me a bit of coin too!
Free, for Mac OS X 10.4 or later.