Last month I opened our living room window one morning to find these neat little nest cells in the frame of the window. Each was made of mud and sand and was about 20mm long. The window must have been left slightly open, leaving a narrow gap for an insect to get in to make them. A little research on the web, and in books, suggested that the maker was probably either a potter wasp or a mason wasp. These put paralyzed grubs or caterpillars into each nest to provide food for their eggs when they hatch out. From the look of it, the wasp had been disturbed before it had finished the cells. One had only just been begun and two were empty (maybe they had hatched out?). However the left-most one does seem to have its opening stuffed with something, so maybe they weren't all hatched out.
Photo taken on 2008-06-29.