Mosquito net. Modern cities may not find use for them but there is a pretty big chunk of world's population for whom this simple item is indispensable. Its cheap. Protects against diseases like malaria, dengue etc. And definitely helps with getting a good sleep.
Many villagers in India build it themselves for the community and i hope it stays that way. I know its is popular in the subcontinent (India,pakistan, bangladesh, sri lanka.) Would love to know if other countries also use them