It's a famous death scream from a movie years and years ago (1951), that was considered so good, it is used in most movies now, but more as an inside-joke. It was even in the Transformers movie last year, when Sam kicked Frenzy's decapitated head. It was mixed to sound a little robotic, but you can still tell it was 'the scream'.
Once I heard about it on some documentary years ago, it just stands out
whenever I hear it in a movie now.

For more on the Wilhelm Scream, check this link for its history:
http://www.youtube.com/watch?v=_PxALy22utc


Then watch these two compilations for just some of the examples of movies the scream has been used in:
http://www.youtube.com/watch?v=cdbYsoEasio
http://www.youtube.com/watch?v=4YDpuA90KEY

And then watch this clip, which has someone intentionally overusing the scream, to get a very amusing result:
http://www.youtube.com/watch?v=PhB1AZCaP6UAnd one not as funny, but still amusing:
http://www.youtube.com/watch?v=kHXRyTwsTQc