I have a Kartmaster.

http://www.kart-a-bag.com/html/kartmaster-hd500.html

I've had it since 1998, and it allowed me to sell my aging Toyota Land Cruiser and go to a Suzuki Sidekick that gets 28mpg on the highway. It's a fantastic thing, folds up to less than 2 feet tall and is incredibly rugged. I've probably loaded it with up to 400 pounds before. Not cheap, but worth it.