Horrible but ubiquitous sound effect:

It all began in 1951, during the post-production of Distant Drums, a Western starring Gary Cooper. In the film, there was a scene in which a soldier was attacked by an alligator while wading through a swamp. As was (and still is) practice, many sound effects were recorded in a studio after filming, then mixed into the final edit.

To fulfill that particular scene’s sound direction -- “man gets bit by an alligator, and he screams” -- the film’s producers called in a voice actor; in rapid, one-take segments, the actor recorded six “pained screams.” Ultimately, scream #4 was chosen as the most appropriate for the scene, and made its debut on the silver screen. (Note: the other recordings have also been used from time to time, but #4 was the most enduring). 

Though there is no conclusive proof of who recorded the scream, several people contend that it was most likely the work of a musician named Sheb Wooley, who is best known for recording the 50s hit, “Purple People Eater.” Written paperwork from Warner Brothers’ Distant Drum recording session list Wooley as a performer that day, and in a later interview, his wife recalled that “He always used to joke about how he was so great about screaming and dying in films.”  

Along with other recordings, the scream was archived in a Hollywood sound effects library. 

Over the next 20 years, the scream was sparingly resurrected for use in Warner Brothers films -- so infrequently, that viewers of Distant Drums never realized they’d heard the same sound effect before. By the mid-70s, it had faded out of use.

Around this time, a young technician named Ben Burtt was tasked with finding sound effects for use in George Lucas’ Star Wars. While digging around in Warner Brothers’ archives, he came across the original recording of the scream. After realizing that the effect had been used in the 1953 Western, “Charge at Feather River” (in which a character named Wilhelm is shot in the leg by an arrow), Burtt dubbed it the “Wilhelm Scream,” and decided it was too good not to use.

When Star Wars debuted in 1977, it included a scene in which a Stormtrooper was blasted off a ledge with a ray gun. He emits, of course, a hearty Wilhelm Scream.

After being featured in Star Wars, the Wilhelm Scream became a bit of a running joke among Burtt and his sound producer buds. After it was re-archived at Skywalker Sound, Burtt used it in the Indiana Jones films (Raiders of the Lost Ark, 1981; Temple of Doom, 1984), More American Graffiti (1979), and Willow (1988), among others.