Fuelly

I came across a website today while looking up car information, it is Fuelly.

The interface is pretty easy though sometimes somewhat glitchy.
Arguably it's a lot of information to put out there about your vehicle and could allow for some inferences in terms of driving habits, have you taken a road trip / done a lot of driving, and other things.
But, I really don't mind sharing this information because I think the aggregate data that this generates in terms of real world fuel economy for a vehicle, and the averages across numerous drivers, is fantastic and does something that staring at the EPA estimates online or off a sticker does not do.
It also gives you information across model years where there were minor tweaks and changes.
In some ways I think it provides a really cool tool for keeping auto manufacturers honest.
Also, it probably looks a lot nicer with the charts and info graphics than the Excel spreadsheet I use to keep track of gas consumption and fuel economy.

I made a profile there for anyone interested (and also so I don't lose track of it / forget about it in the future):
Cagalli Yula Fuelly Profile