If you live in Austin, Baltimore, eastern Iowa, San Diego, Boise, or the Research Triangle there is a study of the gas tax that is going to start soon in these areas. They install a GPS in your car to monitor how many miles you drive as well as a little keypad for you to enter in how many gallons of gas you buy. If you get selected for the study you get $300 up front, $40 per month for 10 months for participating, and $275 for completing the study. Sounds pretty easy to me!
http://roaduserstudy.org/