RE: The World's Most Important Invention: How We Think About Technology
Birth control pill. It fundamentally shifted the traditional limits of one-half of human society, for good and ill. The ability to consistently influence timing of conception has resulted in current dynamic of declining marriages, delaying of child-birth, psychic and social separation of women from traditional roles, psychic and social separation of males from responsibility of conception, psychic and social diminution of fetus into non-personhood, psychic and social shift in conceptualizing marriage outside of child-rearing, etc.
Haber-Bosch process gave humanity the ability to industrialize agriculture, but the birth control pill mitigates the necessity for excess food production with ever-declining birth rates into negative territory. Humanity may breed itself out of existence.
Birth control pill is a good one, but lack of access and various social and cultural disincentives to use it have resulted in a still growing global population. (Birth control in general, including the condom, is a better thing to look at.) It hasn't made any real dent in the birth rate on its own- those few industrialized nations that have slowed their population growth down merely use birth control as one mechanism among many to control growth. The actual causes seem to be increasing standards of living and various (again) cultural and social causes.