I use Gas Cubby and I love it. It's so handy to enter the info when I fill up and then I can send the info to a spread sheet on my imac or MacBook Pro. It will also keep track of service work I have done.
+1 for Gas Cubby. Great app with tons of information. It even goes into detail such as cost per mile, cost per day, average price per fill up, etc. App Cubby • Gas Cubby • MPG (Fuel Economy) and Vehicle Maintenance for iPhone and iPod Touch
Road Trip...almost all the data you could ask for in one screen. (Plus the ability to view the data for specific time spans.)
If you like it, you will upgrade to the paid version of Gas Cubby. The Lite version will only allow you to enter 5 gas records. Then you need to pony up the $$. I have found it worth it, and my costs are about $.05 per mile so far, as opposed to $.23 in our Honda van.