While reading Wired News at the gym, I came across the article: "Cue the Scream: Meet Hollywood's Go-To Shriek".
The scream was from a scene in a 1951 film, "Distant Drums", where a cowboy found himself too close to an alligator and lost an arm in the process.
The scream echoes through film history, finding it's way into film after film, as a sort of inside joke amongst sound professionals in the business. The article even provides a spiffy timeline as to some easily recognizable uses of the clip.
That's just a taste. For more history, check out the Wikipedia article: "Wilhelm Scream".
I'd isolate the scream, but cannot for copyright reasons. However, there's a link in the Wired article (above) and chrisofduke posted a fun compilation on YouTube: "The Wilhelm Scream Compilation".