Markus Kayser's Solar Sinter Project, is essentially a 3D printer using one of the worlds most abundant materials, SAND, and sunlight!
My mind reels at the possibilities of this device on a grand scale. Glass cities and uncovered civilizations buried in the sand....etc....
Video demonstration: http://vimeo.com/25401444
Markus' Website: http://www.markuskayser.com/work/solarsinter/