I've been pondering your millennial/bimillennial question, and I'd like to cheat a bit by giving several answers.
I too offer a vote for the oral contraceptive pill. It is revolutionary for two reasons. First, it makes a quantum leap in the effectiveness of technologies for the control of human fertility — which are found in every known culture and likely date back more than a hundred millennia. The pill and subsequent devices have the potential for a revolutionary impact on the lives of women from puberty to menopause everywhere in the world, allowing women to control their own fertility and thus enabling members of half the human species to control their own adult lives.
In addition, these devices have the potential to save the planet Earth from the ongoing disaster of human overpopulation, with its present and future dire consequences globally of mass poverty, pandemics, warfare and violent confrontations over scarce resources, environmental degradation, and wholesale species extinctions.
My next vote for most important technology of the last two thousand years goes to the gun, or more precisely to a series of European inventions of more efficient killing technologies. The ship-mounted cannon, the Spanish trabuco and the British Snider rifle — to mention just a few weapons from recent centuries — in the hands of members of authoritarian societies (whose populations had exceeded the carrying capacities of their homelands given contemporary agricultural technologies), bent on acquiring new territories, propelled across the Atlantic and Pacific Oceans by ships built according to the most advanced maritime technologies of their eras, effected the European conquest of large portions of the planet's landmass, resources, and human populations. The momentous consequences of the European conquest will continue to play themselves out in every sphere of human life around the globe over the next millennium.
My final vote goes to the revolutionary improvements in hydraulic engineering made beginning in the late nineteenth century that have solved what has for millennia been the single greatest problem of urban life: how to bring clean water in and human waste out of a large nucleated settlement. While the Roman waterworks were brilliantly designed (and their epoch crosses the bimillennial cut-off point of this exercise), improvements in sanitation made only a century or so from the present led, in industrial societies like Britain and the United States, to a revolutionary drop in the death rate from infectious diseases transmitted by fecal contamination of drinking water. These advances in hydraulic engineering have extended human life spans even more than the subsequent discovery of antibiotics.
This technology has diffused only slowly around the globe as it encounters barriers created by unequal distributions of wealth and power. Even so, ironically, our resulting increased longevity, and the increases in population fertility that declines in mortality rates confer when they are unchecked by other variables, contribute dramatically to the ongoing crisis of human overpopulation. This makes the wide availability of advanced contraceptive technology, invented two generations later, all the more critical for the survival and well-being of our species and of the entire planet.