The Wilhelm Scream

This isn't a gag - these are the actual soundtracks to the films. Seemingly a favorite among sound editors, the wilhelm scream has been with us since the 50's and was used in all six of the star wars atrocities, along with hundreds of other movies and tv shows. I learned about it just now, while reading the "trivia" section on the imdb.com page for "howard the duck". See the whole list on wikipedia.