www.cashforclunkersfacts.com
Basically, if you have a car, truck, SUV that gets less than a certain gas mileage you can trade it in on a new car that get better gas mileage. Depending on the difference the government will give you a voucher for either $3,500 - $4,500, even though your car isn't worth that.
For instance my 2001 Isuzu Rodeo is worth about $3000. It gets less than 16 mpg. If I trade it in on a new car that gets at least 26 mpg I get the $4500 voucher. If I trade it in on a new car that gets at least 20 mpg I get the $3500 voucher.
President Obama still needs to sign it (which he's indicated he'll do) and there are some kinks to be worked out, but if you're in the market for a new car and have something older and not worth much, you could do pretty well.