Fuelly is a site that tracks your gas mileage over time, helping you calculate fuel expenses as you drive.
Fuelly was built by Matt and Paul in a little over two weeks during July 2008. The idea started with building a MPG history calculator app, but making it web-based and multi-user so you could compare cars and drivers across the system. New feature suggestions are always welcome in the Forum.
How to use the site
Sign up for free, add a car or motorcycle to your profile, then either keep track of miles driven between fuel-ups (using your vehicle's tripometer) or record your odometer at each fuel-up (you can choose in your settings which way to record mileage).
At http://m.fuelly.com/ we have a mobile version for adding data right from the gas pump. Please view our FAQ item about using Fuelly Mobile here.