Philip Anderson asks the right question: "Why has no one mentioned the printing press yet?"
I mean, doesn't it seem kind of obvious that printing — under which would be subsumed all forms of large-scale reproduction of the written word, from handmade wooden type to the computer and word-processing program I'm using to write this — was the most important invention of the past two thousand years? Printing led directly to mass literacy, democracy, the scientific revolution, cyberthis and cyberthat, and all those other good things.
A more general observation. I notice that most of the responses you included in the email suggest that the most important invention of the past two thousand years, whatever it was, just happens to have happened in the past hundred years. Doesn't this reflect a bad case of chronocentrism, i.e., the irrational belief that one is lucky enough to be living in history's most important era? Given that people have been inventing things all along, isn't it unlikely that all the most important inventions would have happened in one little century out of twenty? Wouldn't it be more logical to expect them to be spaced out randomly over all twenty? Even if the twentieth is a particularly inventive century, isn't it a little myopic to imagine that the one we just happen to be living in is twenty times more inventive than any of the others? Maybe four or five times more inventive, but even that would be a stretch.