GasBuddy is an excellent way to find gas on the cheap. The app provides the closest gas stations to your current location, as well as their location (on a map if you choose) and the current price of gas. GasBuddy also provides a point system for reporting gas prices, as a way to keep their app up-to-date and relevant. Please note that gas prices are only provided for the United States and Canada.
To me, the only reason not to download this excellent tool is if you have it built in to your navigational device/app (i.e. Waze). Everyone wants to save money and sometimes driving 2 more miles will net you a huge discount.
Requires: iPhone/iPod Touch/iPad; iOS 3.0 and up
Download size: 6.3 MB