Updated: I re-did the table with the new data from Edmunds, after NHTSA updated some of their mileage numbers.
Use my handy applet below to look up your car to see if it might qualify for the CARS "cash for clunkers" voucher from the Uncle Sam. This is a list of cars from Edmunds that gets 18 MPG or less and has an expected fair market value of less than about $4,500. Edmunds was kind enough to post the raw data (here) and I massaged it a little and stuffed it into a Google Docs spreadsheet.
You may need to wait a little while for the data to load.
To use the applet, pick your car's age and make from the drop down list.
Update: Edmund's data only goes back to 1990, that's why that 1984 Saab isn't on there. Only cars younger than 25 years old are eligible, so the oldest model year that would qualify would be a 1984.