Bookkeeping
I know that I've seen it claimed that the ancient Babylonians invented double-entry bookkeeping.
Thinking about the original question, feeling that even "the computer" wouldn't be the third great invention, let alone the relational database... led me to think that perhaps one candidate would be modern place-value notation - the Hindu system of writing numbers that was transmitted to us by the Arabs. That greatly simplified working with numbers by pencil and paper.
Of course, the germ of that idea was contained in the abacus, perhaps the first mechanical aid to computation.
Biting the hand that feeds IT
