My immediate response (without even thinking) was the contraceptive pill. My mother had six children in five and a half years and it was only the invention of the pill that saved our family from becoming a mini-nation-state in its own right. But since Colin Blakemore has already described so well its immense importance, let me suggest another "invention" — electrification.
Why electrification? For a start, one of my most vivid childhood memories is of my mother seemingly spending endless hours washing nappies and clothes by hand. The electric washing machine and other electric home gadgets (vacuum cleaners, fridges, food processors et cetera) have freed billions of women from the endless drudgery of heavy-duty housework. By bringing us light and heat and power on tap, electricity has truly transformed life — not just in the home, but in almost every industry. Modern manufacturing would be impossible without electricity. Ditto the modern office. The ability to literally transport power is, I think, the most revolutionary technology to come out of modern science. And of course, it is the ability to transport electric power at the micro level that has made possible silicon chips, and the attendant computer and information revolution. Far more than Einstein and Bohr, Faraday and Maxwell are the true "heroes" of the modern technological world.