Wikipedia
Wikimedia publishes full database dumps of Wikipedia, Wiktionary, Wikiquote, and the rest of its projects at dumps.wikimedia.org. You can download a single language edition or the lot, as raw wikitext or as parsed HTML.
For RAG, the English Wikipedia article text is a common starting point: it is broad, well structured, and cleanly licensed. The compressed English text dump is around 22 GB. Many pre-processed, article-per-record versions are also published on the Hugging Face Hub if you would rather skip the parsing step.
The text is licensed under Creative Commons Attribution-ShareAlike, which means you can reuse it as long as you credit Wikipedia and share any derived text under the same terms.
Related sources
DBpedia
A knowledge graph built by pulling the structured parts of Wikipedia, mainly the infoboxes, into machine-readable data. It holds billions of facts about people, places, organisations, and more as RDF triples, small subject, predicate, object statements, which you can search with the SPARQL query language.
Freebase
A collaborative knowledge base once run by Google, now retired but still available as downloadable data dumps. It holds structured facts about millions of entities, and much of its content has since moved into Wikidata.
OpenCyc
The open release of Cyc, one of the oldest attempts to hand-build common-sense knowledge for machines. It holds hundreds of thousands of concepts and millions of assertions about how the everyday world fits together. Now archived, but the data is still available.
Wikidata
A free, collaborative knowledge base that holds structured facts for Wikipedia and the other Wikimedia projects. More than 100M items, each one machine-readable with statements, relationships, and identifiers, so you can pull clean facts instead of parsing article prose. Released into the public domain under CC0.