Freebase
Freebase let a community build a shared graph of entities and the facts that connect them, covering people, places, films, and much more. Google acquired it in 2010 and used it to help seed its own Knowledge Graph, then wound the public service down in 2015 and handed the data to the Wikidata community.
The service and its live API are gone, but the final data dumps remain available for download, published as RDF triples: small statements of the form subject, predicate, object. You can still load these into your own store if you need the historical snapshot or want to trace how facts were modelled.
For most new work, Wikidata is the better starting point, since it absorbed a large share of Freebase's content and stays actively maintained. Freebase is worth knowing mainly for legacy datasets and research that was built on it.
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.
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.
Wikipedia
Complete database dumps of Wikipedia and the other Wikimedia projects, in every language, refreshed roughly twice a month. The most widely used knowledge source for RAG systems, and an easy first corpus to build a retrieval pipeline on.