Mashups are great ! GasBuddy.com is a a great idea too. On their website you can input your city or zip code and it will display a local map with current gas prices from several stations. What I really like about GasBuddy.com is they share this data with developers. I also found a great wordpress theme that is styled to display flawlessly on the Iphone. Coupled together, along with a few CSS tweaks- and voila a really useful Iphone web app !
http://www.apple.com/webapps/searchtools/localgasprices.html
