I propose that the most important invention in the past two thousand years is: "Languages". If you take a look at the proposals you have received (or will) so far: the contraceptive pill, the scientific method (whatever that means), the quantum theory, and so on, they could not have even been thought out, let alone conveyed, without the aid of a language. I do not mean a language in particular, but all the languages, dead or alive. Of course one tends to think that live languages deserve the credit, but without the so-called "dead languages", such as Latin, the live ones simply would not exist. If one accepts that language is the most important invention in the past 2000 years, one has to concede that the "Human Brain" is the most important inventor during the same period.
In my opinion, the printing press comes second to languages as the most important invention in the past 20 centuries; this puts Johann Gutenberg (c.1400-1468) as the second most important inventor of all, since one can easily pinpoint him as the Father of the printed letter. Without a (written) language, specially when it conveys concepts and feelings, all cultures — scientific, literary or otherwise — would be all but a conceptless matter. The Third Culture simply could not breathe.
One can speculate ad nauseam about which language in the current state of world affairs, including the Internet, is the most important one of all. I have some ideas, to theorize about them, though, is beyond your original question.