In the days before the interwebs, when last-minute tickets between SFO and LAX were still cheap, I'd check the travel section of the classifieds. In the summer months, there always seemed to be a car rental agency willing to give you a car for four days, including a free tank of gas, if you would just pick it up at SFO and return it to LAX for them.

You got a free car for the weekend in L.A., a free drive down, and only had to pay local expenses plus buy a cheap one-way flight home. They'd even let you park your car in their rental lot for free over the weekend. Sweet deal. Did that many times with Hertz.

For some reason, people seem to like to start their vacations in L.A., then drop the rental in S.F. or Vegas. They need to get the cars back somehow, and it costs them real dollars to have them trucked back in.