Gas.App is a very useful application to help iPhone users find the closest cheap gas station. You just enter the zip of your location and Gas.app will give you prices for regular, plus, or premium. Location of different gas station is listed and you can click on the map to see it the location on Google maps. Direction will be integrated soon and will certainly make the application more exciting to use. Gas.app uses SimpleJS library with PHP to retrieve gas prices.

iPhone link: http://www.gasapp.com/new.html
Website : http://www.gasapp.com/