GeoNames
GeoNames is a gazetteer, a directory of geographic names, that pulls together data from national mapping agencies and other official sources into one consistent dataset. Every entry has a stable identifier, latitude and longitude, a feature type (city, mountain, river, and so on), and links up the administrative hierarchy from country down to local area.
You can download the whole database as tab-separated text files for bulk use, or call the web services for lookups such as reverse geocoding, nearby places, and name search. Alternate names and translations in many languages make it handy for matching place names written in different scripts.
For RAG, GeoNames is a reliable way to link a mention of a place to a precise location and its context. It is widely used to enrich datasets, disambiguate place names, and power location search.
Related sources
Natural Earth
A public domain map dataset built for cartography, offered at three scales: 1:10m, 1:50m, and 1:110m. It bundles cultural data (borders, cities, roads), physical data (coastlines, rivers, lakes), and raster imagery, all ready to drop into a map.
OpenStreetMap
A crowdsourced map of the world, built and maintained by a community of volunteers. It holds detailed geographic data on roads, buildings, land use, points of interest, and much more, which you can download as a full planet file or as smaller regional extracts.
Overture Maps Foundation
An open map data project run under the Linux Foundation by a group of industry members. It combines and harmonises data from OpenStreetMap, Microsoft, Meta, and others into consistent, interoperable map layers you can build on freely.