Meet the Kyoto Box. Technically, it's two boxes, one inside the other. Cut the top off the big box and put a piece of clear plastic in its place. Line the inside of the big box with foil. Paint the inner box black. Put a pot of water inside the little box, put the whole thing in the sun and it'll boil.
No, probably not an alternative for most. But in Africa, it's better than cutting down trees for a fire. And it allows boiling water to purify it.