OpenStreetMap

From   
Created in 2004 by a British academic, OSM is a sort of Wikipedia for maps, with volunteers contributing geographic information into a centralized database, which can then be accessed by anyone for free.

OSM was mostly the domain of academics and hobbyists until 2012 when Google announced it would start charging more businesses for using Google Maps. Since then, companies from Amazon to Meta to Snapchat have built mapping products on OSM.
— Jason Tanz (2023) "Lyft’s secret plan to take control of its maps — and its future Lyft Rev

Backend

OpenStreetMap is an interesting project because they actually started on MySQL and then they started having performance problems with MySQL. So they decided to consolidate on Postgres. So most of their editing stuff is just using raw Postgres and the tile server use PostGIS.
—Regina Obe (September 8, 2023) "Why people care about PostGIS and Postgres" Path To Citus Con