Worldchanging and Inhabitat have posts about using trees to grow the walls of a house. The technique, called pleaching, bends and weaves the trees together into a structure, which can even provide fruit for its inhabitants. The walls are then covered with cob (a mixture of mud and straw) for insulation and waterproofing. You end up with a genuine treehouse. Talk about fun!
I wonder whether these kinds of houses could prove beneficial in the developing world.