These are the cells of a Bald-faced hornet nest. The nests are abandoned in autumn and eventually fall down, or are torn apart by birds or animals looking for any edible remains inside. It struck me today, when I found this one, that the shape of the cells is quite remarkable. And look how intricate and how even they are! All this made by the wasps chewing wood fibres and slowly and laboriously building these large structures in which to rear their young. Really, it is remarkable.
For others in the Monochrome mood: http://form.pbase.com/viewtopic.php?f=16&t=46276