Here's a handy miles per dollar calculator that you can use to determine how much you spend commuting each day. At current gas prices, my commute costs me about $2.50 round-trip. The calculator would have been even cooler if you could input your car type and have it amortize the car's value over your mileage.
(On a side note, I have a friend from Canada where they use "kilometers" and "litres" to measure things, and yet their cars are still rated by mileage. Just to make things more confusing, the units they use are "litres per kilometer".)
(HT: Sound Mind Investing (registration required).)