Joshua James at 2009-06-26 07:50:15:
One of the most frightening climaxes ever, because we wanted to know what was in the box, but at the same time, we didn't ... after two hours of gruesome deaths, the one that horrified us the most is the one we don't see, we don't see what's in the box ... which makes it all the scarier.
Interestingly enough, it only happened this way because Fincher got sent an early draft of the script by mistake, originally the studio had a more conventional ending to it ... Fincher got a draft with this ending, called his agent and said, "are they really gonna let me end with her head in a box? If so, count me in!" and he got Pitt to champion the ending, too.
Great film, a really great film.