Adrian and Doug got it spot on while many others correctly guessed that it was a cocoon of some kind. Well done and thanks for playing everyone.
It is the egg sack (proper name: ootheca) of a mantis. The eggs are layed in a frothy mass that hardens as it dries, and is about 1 inch in length and would contain over a hundred eggs. Mantids hatching from it are about the size of a medium ant for which they are often mistaken. The pictures below is of a mantis which is about to shed its skin, hence the brown color, as they are usually green.
