The Bat bomb:
http://en.wikipedia.org/wiki/Bat_bomb
This one, by the americans, actually worked too well. It involved strapping bombs to bats, putting them in cold induced hibernation and then dropping them out of a plane as a bomb. The bats would wake up, fly out of the bomb, climb into attics and rafters and the incindierary device would explode causing thousands of fires everywhere.
In it's one test, some of the bats escaped, flew into the rafters of the hanger and burnt it down. Great stuff.