The Wilhelm scream is one of the most well-known sound effects of all time. It has been used in movies, TV shows and video games for over 50 years. The scream was recorded for the 1951 film Distant Drums and used in a scene where a man is being torn to pieces by an alligator.
The identity of the actor who voiced the Wilhelm scream is unknown, but the most likely source is considered to be Sheb Wooley, known for the song "Purple People Eater."
The Wilhelm scream was used in all six Star Wars movies, Poltergeist, all three Pirates of the Caribbean films, Monty Python and the Holy Grail, Reservoir Dogs and all four Indiana Jones movies. Did you catch it in the library scene in Indiana Jones and the Kingdom of the Crystal Skull?
Check out wikipedia + hollywoodlostandfound.net for more.