@Spencer @Dr. Elizabeth Sawin✨ My absolute favorite is the distribution system allowing running water for all. This isn't very old, the British mandate to have running water in every household dates from the XXth century. The grand-parents of my own generation definitely remember a time when some people around them had to get water from a remote source.

Next on my list is the sewer system, which allowed high density cities to thrive without choking on their own excrements. Neither of them are particularly complex technologies to grasp, but the coordinated efforts they both required to be implemented is for me a marvel to behold.