Aerogels are the lowest density solids on earth, typically between 80%-99% air.  They also have unique porosities and very high surface areas.  They were first popularized by NASA through the insulation of the MARS rover.  Since then, they have been utilized by Oil and Petrochemical companies for high-performance insulation.  They have new developments in energy storage, cryogenics, thermal management, light-weighting, and acoustic dampening.