I could swear I've already written this post, but I can't find it in the archives, and it's so important it bears repeating anyway. Thanks to Denny for reminding me.
You may not know it, but you have probably heard this sound effect dozens, maybe hundreds of times:
This scream was first recorded for the movie Distant Drums in 1951. The scream, originally titled, appropriately enough, Man being eaten by alligator, next appears in the 1953 film The Charge at Feather River. It is from this movie that the scream gets its name - a character called Wilhelm makes the noise after being shot in the leg with an arrow.
The scream was worked into the soundtrack of B monster movies over the next many years - a sort of in-joke, a way for sound designers to leave their mark on a movie. It somehow manages to show up twice in the Judy Garland film A Star is Born, probably my favorite use of it just because it's so friggin random.
The scream got a revival when Benjamin Burtt Jr., sound designer for just about every Lucas film, stuck it into the first Star Wars movie, and then every Star Wars movie after that, as well as the Indiana Jones movies, and, I suspect, every other movie he ever worked on.
It then became a phenomenon, with sound designers sneaking it in where ever they could. Often, directors would catch it and demand it be taken out, so people began slipping the scream in all sneaky like.
What I love about the scream is that, once you know to listen for it, it sounds absurdly out of place. Like, is that really a sound that a Storm Trooper would make? An orc in Lord of the Rings? Buzz Lightyear?
And I totally get it. At my job too, I write all of these things all day, and they're dry and clinical and there's nothing of me in them. I wish I could find the visual equivalent of a Wilhelm to sneak into my Help documents.
By the by, there's a good chance that the first guy to utter the Wilhelm scream was none other than Sheb Wooley - the guy who first recorded Flying Purple People Eater. By the by the by, does the Purple People Eater eat purple people, or is he himself purple? I've never been able to figure that one out.
Here's a little compilation for your viewing pleasure.
Info from an On The Media story, supplemented with Wikipedia.