The Wilhelm Scream

From Blastr.com:

The scream itself was first recorded in 1951—by singer and actor Sheb Wooley—for a film called Distant Drums, in which a soldier is bitten by an alligator. Twenty-some-odd years later, sound designer Ben Burtt found the recording (in a can marked "man eaten by alligator") and was so taken with it that he mixed it into a scene in Star Wars: Episode IV—A New Hope.
Burtt would then proceed to incorporate the Scream—named after a character that uttered it in 1953's The Charge at Feather River, Private Wilhelm—in most every film he worked on for George Lucas or Steven Spielberg, including all the Indiana Jones flicks.