The greatest innovation in my field of biology? No contest - the discovery of microbes. Yes, yes, I know we are all excited about telescopes - and recent developments like the Hubble are marvellous - but telescopes can only reveal more and better examples of the heavenly bodies that we have always seen.
By contrast, microscopes reveal living universes of microbes, which you cannot otherwise know. Until 1674, nobody knew they were there - yet now they are among the most important aspects of our modern lives.
Microbes were discovered by the Dutch draper and town official Antony van Leeuwenhoek. He began his microscopical career at the age of 40, and then went on to devote half a century to founding modern microbiology. He observed cells, nuclei, spermatozoa, and so much more - and neatly disposed of the idea of spontaneous generation long before Pasteur. This was the most wonderful revelation in the history of the biosciences. And he did all this with little home-made microscopes and hand-ground lenses that he made himself. All this happened three-and-a-half centuries ago. Truly remarkable.