I'm gonna go with microscope. Visualizing microorganisms led to an entirely new understanding of disease. It also led to an understanding that the world was made up of much more than we can see.
By Elaine (not verified) on 14 Oct 2009 #permalink
I spent a while chasing ideas around as to what scientific invention has had the most influence. Since one of the properties of science is that everything in its purview can be traced back to something earlier, typically collections of somethings earlier, one could say fire, farming, or the wheel, or Roman Road building, or Galieo pointing his telescope to the skies, or the observation of semiconductor amplification (as it happens, coincident with my date of birth, 17 November 1947). I felt trapped in an infinite regression.