Markus Kayser has a very interesting project, called SolarSinter, where he is using the energy from the sun to fuse sand into glass. This is very thought provoking as it utilizes two very ubiquitous things solar energy and sand. While the printer does not has very high precision at the moment, it does appear to have potential for additional automation and refinement.
A very interesting thought would be using these on a planet, like Mars, to fabricate buildings from local materials and solar energy. The biggest question would be if the sand on Mars would sinter.