We've seen pigeons
that monitor pollution and cement
that eats pollution, but for eye-catchiness and do-goodedness, it's tough to beat Soo-in Yang and David Benjamin's River Glow project, which provides a light show as it keeps an eye on water quality. Built with a budget of $1,000, the current bathtub-bound prototype system consists of an LED connected to uncoated fiber optic strands, which gets triggered when the pH sensor detects changes in water quality. What's more, the whole thing's apparently powered by a single AA battery, which is recharged using floating strips of thin film photovoltaics. Presumably, that'd be scaled up before it gets let loose in some larger waterways (as seen above), although it's not clear when or if that'll happen.