More Po-2's were built than any other kind of airplane...ever. It was originally known as the U-2, and used largely as a crop duster and training plane until World War II began, after which it was used for anything the Army could think of. Its closest brush with fame was its service with the "Night Witches", women pilots who harrassed German troops on daring nocturnal raids. The Luftwaffe found it frustratingly difficult to shoot down, as it was nearly invisible to radar and slower than the stall speed of their fighters.
Cool stuff for Po-2 lovers:
One of the most unique and engrossing narratives of the Soviet Air War you'll find is Night Witches, an account of the exploits of women fighter pilots who fought on the Eastern Front. Many of them flew the antiquated Po-2 into combat, and have quite a few hair-raising stories to tell about it.