At the beginning of the year and started keeping track of the cost (and amount) of gas I use by entering my receipts into a Google spreadsheet.
When I graphed out the price of gas over the last few months it is quite eye opening on how much I am paying:
This is one of the reasons I love keeping track of data. As I fill up a the tank every week or so I don't really notice the price jumping that much but looking at the data it becomes clear as to where my money is going!
I also find it interesting to track my gas mileage. (I actually noticed when Atlanta switched to the Summer blend because my fuel economy went up 2mpg!)