The world's least dense solid is an aerogel produced by Lawrence Livermore National Laboratory, Livermore, California, USA. With a density of just 1mg/cm3 (0.25 grains/in3), this aerogel is lighter than air itself (1.2 mg/cm3, 0.3 grains/inch3) and is virtually invisible to the untrained eye. Aerogel with this density was first created on 10 April 2003.
note, the '3' after 'cm' and 'in' should be superscript - as in cubed.
Comments below may relate to previous holders of this record.