Though some may dispute what actually constitutes a "hamburger", the town of Seymour, Wisconsin is so sure the hamburger was invented there, they even have a Hamburger Hall of Fame (built to honor Seymour native and hamburger inventor Charlie nagreen) to prove it!
The story goes that then 15 year old Charlie used to sell hamburgers from the back of his ox-drawn cart at the Outagamie County Fair way back in 1885. His inspiration for the "invention" came when he flattened meatballs and placed them between two slices of bread. Many who knew Charlie referred to him as "Hamburger Charlie". Charlie even had a jingle to accompany the sale of his burgers:
Hamburgers, hamburgers, hamburgers hot; onions in the middle, pickle on top. Makes your lips go flippity flop!
more:
History of Hamburgers, Hamburg Steak, What is a true hamburger/ Meat Choppers, Cheeseburgers