Rensselaer Polytechnic students Peter Zummo and Matthew Naples have developed a modern version of Heineken's Wobo (world bottle-here in TreeHugger and more detail in Inhabitat) of 45 years ago; a plastic bottle that can be filled with sand and locked together like Lego to build housing. Their struggle to patent and bring it to market is covered in The New York Times Education Life section.
Popular Science covered the bottles back in April:
Lego-like though they are, the bottles can create structures far sturdier than a toy--and do so essentially for free. Filled with dirt, sand, or another widely available material, the bottles become heavy enough to act like regular bricks. Build a house from them, and the sand or other filler would serve as an insulator to keep rooms warm or cool. Meanwhile, the plastic would make the house waterproof and thus more permanent than the plain mud huts often found in the poor countries.