Given the amount of self-reference in the answers so far, I'm tempted to nominate this very discussion list as the greatest invention of the past two thousand years, and hopefully out-meta all the other contenders.
I think part of the problem here is the fact that inventions by nature are cumulative, and so when asked to pick out the single most important one, you're inevitably faced with a kind of infinite regress: if the automobile is the most important invention, then why not the combustible engine? (And so onŠ) In that spirit — and in the spirit of nominating things you happen to be working on professionally — I'd nominate the ultimate cumulative invention: the city. Or at least the modern city's role as an information storage and retrieval device. Before there were webs and telegraphs making information faster, there were cities bringing information physically closer together, and organizing it in intelligible ways. It's not a stretch to think of the original urban guilds as file directories on the storage device of the collective mind, combining disparate skills and knowledge bases and placing them into the appropriate slots. (Manuel De Landa has a wonderful riff on this in the first section of his new book, A Thousand Years of Nonlinear History.)
But of course, the city isn't an invention proper, at least in the conventional way that we talk about inventions. It's the sum total of multiple inventions, without each of which the modern city as we know it might not exist.
I think what this discussion makes clear is that we need a better definition of "invention"!