The most important invention of all time was the interrogative sentence. i.e., the asking of questions.
However, the original request was for the most important invention of the last 2,000 years, not of all time. To that I would say, space travel.
Of course, it may be centuries before we know the full consequences of space travel.
Links:
[1] http://edge.org/annual-question/what-is-the-most-important-invention-in-the-past-two-thousand-years
[2] http://edge.org/inthenews/what-is-the-most-important-invention-in-the-past-two-thousand-years
[3] http://edge.org/contributors/what-is-the-most-important-invention-in-the-past-two-thousand-years
[4] http://edge.org/responses/what-is-the-most-important-invention-in-the-past-two-thousand-years
[5] http://edge.org/print/response-detail/10534
[6] http://edge.org/memberbio/reuben_hersh
