The following excerpt is from "The Book of Totally Useless Information" by Don Voorhees:

Who was "Uncle Sam?"

When we picture Uncle Sam today, we picture a man with a long white beard, red-and-white striped pants, and a blue jacket. There was an "Uncle Sam," but he looked nothing like this, although he did wear a top hat and he did come to be a symbol of the U.S. Government.

Sam Wilson supplied the U.S. troops stationed around Troy, N.Y. with meat during the War of 1812. The meat shipped to the soldiers was stamped "U.S." for the United States. One day, when a federal inspector was checking the meat at Wilson's store, he was told by a witty employee that the "U.S." stood for "Uncle Sam," which was Wilson's nickname. It was not long before all federal supplies were said to belong to "Uncle Sam."

While this story was thought to be contrived, an 1830 newspaper article quotes eyewitnesses who swear it's true. In the early 1960s, Congress made it official, proclaiming Sam Wilson the original Uncle Sam.