Personal aircraft, small enough to fit in a garage with the wings folded. Especially the one you could take the wings off at your destination and drive them into town. How many inventors have announced theirs in magazines, set up to sell to an eager populace, and went broke? How often have we read about them in science fiction, only to have them never materialize?
True, over the years they have tended to become anti-grav cars, skimmers, jump-cars, and lose their wings, but why are we all still stuck in rush hour, only dreaming of hitting the button and leaping skyward out of the jammed traffic?