The most accessible general-purpose map app for end users (Android and iOS) is written in C/C++. This is pretty high on the cool and useful scale: https://organicmaps.app/

Both map renderers Tangram-ES and Maplibre-GL - https://github.com/tangrams/tangram-es and - https://github.com/maplibre/maplibre-gl-native are also written in C/C++.

Finally, most routing software, such as OSRM or Valhalla are written in C/C++.