The Wilhelm scream is a frequently-used film and television stock sound effect first used in 1951 for the film "Distant Drums". The effect gained new popularity (its use often becoming an in-joke) after it was used in Star Wars and many other blockbuster films as well as television programs and video games. The scream is often used when someone is falling to his death from great height.
Watch, listen, and smile...
It's remarkable to me how I won't recognize recycled sounds in movies and TV shows, but will almost always notice a reused piece of music score in a trailer for a new movie or TV series and call *fail*.
Still don't know why its called the 'Wilhelm' scream though.