Balsa Wood

Balsa is famously a very light wood; it was used in the frame of the De Havilland Mosquito WWII combat aircraft, and also by Thor Heyerdahl to make his raft Kon–Tiki.

It comes from a tree with the scientific name Ochroma pyramidale, native to Central and South America. Most sources (including Wikipedia) seem to stop short of calling it the world's lightest wood; I did however find one site that was prepared to commit itself, giving its density as about 0.12 grams per cubic metre. (This site gives the tree's genus as Kapocaceae, which I was able to find a reference to anywhere else on the Internet. According to Wikipedia, 'kapok' is a name used in English–speaking countries for the Ceiba pentandra tree, and the cotton–like fluff obtained from its seed pods. This is also native to Mexico and Central America, where it's commonly known as ceiba.)