(Not so obvious) inventions that changed the world:

Double-entry book keeping,
The Gregorian calendar,
the modern emergency room in general...

Dr. John Snow devised the science of epidemiology: A Cholera epidemic struck in London in 1854. Using maps and statistics, Dr. Snow figured out that a certain Soho water pump handle was spreading the contagion-refuting the miasma (vapor, or bad air)theory and developing a germ theory explanation..untold millions of lives have subsequently been saved by proper sanitation and clean water.