There's a novel, simple idea spreading in the Philipines - using bottles of water as skylights. The concept is staggeringly simple - slum dwellings are dark, and need light, even in the daytime. Though they have electricity, it is the most expensive in the area, so they can't afford to use powered lights. Instead, they use water. A two-litre bottle of water, with a little bleach to prevent algal growth, is set into a section of metal sheeting, which in turn is set into the roofs of dwellings, workshops, even chicken-coops. The water-bottle refracts the light in all directions, instead of letting it shine through in a single beam, and provides as much illumination as a traditional 60W bulb. OK, so this is old news to some people, but it's new to me. For more information, have a look at the Litre of Light website, or just have a poke around Google and YouTube.