Practically all of the free map services use OSM.

Also PROJ

Not an app but open street maps is a wonder of the world

I second QGIS sooo much. I would even count it as an app because its shipped integrated (same as blender).

Its ridiculous how many people use QGIS and what amount of responsibility they carry. Its like actually the pro option in the gis space as its much more flexible than the proprietary counterparts.