Cell phones are common in the developing world (97% of Tanzanians have access to one), and they can be a big boost to environmentalism, from tracking air quality to operating irrigation systems. Here's another example of what they can do: It's an inexpensive microscope, built around a cell phone.
The CellScope is the work of the Fletcher Lab at UC Berkeley. A 3D printed casing holds the phone in position so that the camera takes a photo through a magnifying lens. The idea is very similar to what a Finnish research group came up with; the CellScope is bulkier.
The most important applications relate to health care. Microscopic images of blood samples could be used by doctors (or sent to doctors in a remote location) to identify diseases like tuberculosis. The CellScope could evaluate drinking water quality, and also be used for educational purposes. Beyond the developing world, the Fletcher Lab talks about creating loaner kits for schools and community organizations, so more people have access to microscopes.
It's a great example of how sustainable concepts are often embedded in affordable, low-tech design that makes life in the developing world healthier. Using a cell phone as the crux of the microscope saves the materials that would be used in its place. It reduces the weight of the unit (presuming everyone uses their own phone), so shipping the device requires (a little bit) less fuel. And it takes advantage of something most people already have - a cell phone - to make their lives better and greener.