ipstack - ipstack is a free, real-time IP address to location JSON API and database service supporting IPv4 and IPv6 lookup.
QGIS - QGIS is a desktop geographic information system, or GIS.
ipapi - Web analytics with IP address lookup and location API
what3words - Geocoding system for the simple communication of locations with a resolution of 3 m
ipgeolocation.io - Free IP Geolocation API and Accurate GeoIP Lookup Location Database
GRASS GIS - GRASS GIS is a free Geographic Information System (GIS) software used for geospatial data management and analysis, image processing, graphics/maps production, spatial modeling, and visualization.