There are aluminium and copper foams under metals, expanded polystyrene under organics, and what is called reticulated vitreous carbon under nonmetals. Recently I posted balsa wood, not yet in the compendium, which while not a foam has a low enough density to be used as a filler: childrenofadeadearth.boards.net/thread/1534
If you find nice datasheets on any other foams, I can work on modding them in.
Where are you getting 'half the weight and all the strength'? Ultralight materials like these do have high moduli and often retain more strength than one would expect for the reduction in weight, but there is some sacrifice. They often come in at well under half the weight of the base material, around a tenth, though different grades vary a lot.