Of course you can always take your calculator or make your own spreadsheet to calculate how much commuting is going to cost you. Nevertheless, if you want a neat online application that makes all the calculations for you using the EPA's ratings for 2008 models, check out MpgGenie.com.
The Genie can go more than just calculate fuel costs. You can drag different models and even different types of vehicles into the Genie to establish a comparison between different models, something that can be helpful if you're planning to buy a new car. Remember, though, that such calculations are dependent on EPA figures and, as the old ads used to say, "Your mileage may vary".
[Source: MPG Genie, thanks to John for the tip]