I use a GM Mastercard. It gives me 5% back on all purchases to put towards the purchase of a GM car.
I don't charge a whole lot, but the money you earn per billing cycle is good for 7 years from that billing cycle, so I have plenty of time to build up a significant amount. Granted, they have a cap on how much you can use for a given car (different cars have different amounts, but its usually around $1500-2000), but I'm going to be needing a new car soon, and every little bit helps, eh? The website is
www.gmcard.com if you want to take a look at it.
After I use the earnings to buy a car, I'll probably switch to a different rewards program card, though.