If you've got an older car that gets less than 18 MPG and isn't worth shit, but want to trade it in for a new car then the new C.A.R.S. program from the gov't can help:
http://www.cars.gov/
It's a bit convoluted but if you get a brand new car (never titled) with at least 4 MPG higher (IE minimum of 22 combined MPG) you get $3,500 trade-in (and then they crush your old car). If it's 10 MPG higher you get $4,500 trade-in value for the new car. If you're trading in for a truck, van, or SUV, then it's only 2 MPG higher and minimum of 18 MPG you get $3,500, and 5 MPG higher or better then $4,500.
This is better than the original proposed one which was just for American-made cars, because I think figuring out what counted and what didn't gave them a serious headache. Foreign company cars can be assembled in the US, etc.
So I'm looking at my 1997 Chrysler LHS which according to
http://www.fueleconomy.gov gets 18 combined MPG and it looks like it doesn't qualify for a $3,500 trade-in for the CX-9 I was looking at. The CX-9 FWD gets exactly 18 MPG (so the AWD version doesn't qualify ever) but it isn't at least 4 MPG higher than what I have. If I had a car with shittier gas mileage then it would. Bummer.