The Wilhelm Scream Compilation is a wonderful piece of movie geekdom. It all starts with a character named Wilhelm in The Charge at Feather River (1953) — sort of:
A scream used for the character Pvt. Wilhelm (recorded originally for Distant Drums (1951)) has since been reused in many films, including the Star Wars and Indiana Jones films.
More on the origin, from Distant Drums (1951):
This film contains the first known instance of “The Wilhelm Scream” (a sound effect of a man screaming, since used in over 70 other movies). During a scene in which the soldiers are wading through a swamp in the everglades, one of them is bitten and dragged underwater by an alligator. The scream for that character was recorded later. Six short pained screams were recorded in a single take, which was slated “man getting bit by an alligator, and he screams.” The fifth scream was used for the soldier — but the 4th, 5th, and 6th screams recorded in the session were also used earlier in the film when three Indians are shot, one after another, during a raid on a fort. Although the “signature” or “classic” screams, takes 4 through 6 on the original recording, are the most recognizable, all of the screams are referred to as “Wilhelm” by those in the sound community. Ben Burtt, sound effects designer on Star Wars (1977), named it “Wilhelm” after the character that let out the scream in Charge at Feather River (1953). He discovered a file at Warner Bros. for this movie, which contained paperwork that was left over from the picture editor when the film was completed. One of the papers was a short list of names of actors who were scheduled to come in to perform various lines of dialogue for miscellaneous roles in the movie. After reviewing the names and even listening to their voices, one person seemed to be the most likely suspect: Sheb Wooley. Sheb played the uncredited role of Private Jessup in “Distant Drums”, and was one of the few actors assembled for the recording of additional vocal elements for the film. It is very likely he was asked on the spot to perform other things for the film, including the screams for a man being bitten by an alligator.