The Wilhelm Scream is one of the most famous and recognisable stock sound effects used in cinema. I wrote an article about the history of the scream for New Empress magazine called Sheb Wooley: King of Screams.
"A Stormtrooper is shot by Luke Skywalker and falls into the depths of the Death Star. Buzz Lightyear is knocked out of a bedroom window. Indiana Jones crashes into a band in a nightclub. Legolas climbs a Mumakil and throws one of its riders to the ground. Chances are you’ve viewed all of these popular movie moments more than once but did you recognise their common denominator? A high-pitched, drawn-out, comedic scream that unites over 200 films, TV shows and video games over the past 60 years including numerous Hollywood blockbusters."
You can read the full article here. Also, check out this awesome YouTube montage of clips featuring the scream: