Here is a compilation of it.
Edit: I guess I just associated it with someone falling. The compilation shows it's more diverse than that.
Sheb Woley is the actor credited with the origin of the "Wilhelm Scream". It was either part of the 1951 movie Distant Drums or the 1953 movie The Charge at Feather River (the origin movie is still being debated, but the voice is generally agreed upon).
It became part of a Warner Bros collection of stock sound effects and then forgotten about.
It was then later discovered by Ben Burrt (Sound Designer) on studio reel labelled "Man being eaten by alligator". Ben went on and included it in 1977 movie Star Wars, and the 1981 movie Raiders of the Lost Ark, at which point it became a bit of a sound designer "in joke" and started showing up in many many movies.
IMDB tracks the list of movies the Wilhelm Scream shows up in (currently at 361)
There's also another youtube video with the History of the Wilhelm Scream from Hollywood Lost and Found.