I've gotta go with movable type.
Even though it predates 1000 AD by a long ways, I'll go with written language. Once that took hold, large-scale commerce appeared, taking villages out of isolation and creating larger civilizations. Plus, any knowledge gets written down and documented, creating
accumulated knowledge, which made new inventions, sciences, and higher education possible.
The invention of machines that print movable type would not have been possible without written language.
Of the last thousand years,... hmm...
Gimbal-mounted clocks on ships are one thing. They allowed navigators to accurately track and map their courses, and enabled complete exploration of the whole planet for the first time ever.
Optics. Specifically, micro- and "macro"-optics, from microscopes to telescopes. They literally changed our perception of the universe. The tangible discovery and study of bacteria & viruses completely transformed medicine. The accurate mapping of the other planets and their moons, likewise, completely transformed our concept of the entire solar system, and by extension, the physical universe as a whole.
That now makes me think of Sir Isaac Newton's calculations that created modern physics. We wouldn't have gone to the Moon without that knowledge; we wouldn't have satellites for weather study or communication, either.
Anybody remember the documentary TV series "Connections"? James Burke explained the ties between all kinds of ideas & inventions in a truly enlightening way. I mentioned the gimbal-mounted clock because of one of his episodes.
http://en.wikipedia.org/wiki/Connections_(TV_series)