Earthen solar cookers that provide boiling water are mostly being used in poor and developing nations but could be a life save in a shtf scenario. Basically a hole is dug then lined with reflective scrap like broken mirrors and such which reflect the sunlight to the bottom of the hole where you can put gallon jugs painted black and filled with water. This will cause the water to come to a boil and then you could drink it or cook with it. The article talks about how many of earths people are ill from waterborne dieases. If a shtf scenario this could happen very quickly. With all the tornadoes, hurricanes, flooding and other storms that cause water to be contaminated or limited this could provide drinking water so you could avoid becoming ill. According to Appropedia if built according to their specifications you could bring 10 one gallon containers of water to boil per hour (depending on sunlight of course).