OpenCyc

OpenCyc is the freely released portion of Cyc, a project started in 1984 to write down the everyday common sense that people never bother to state out loud: that dropped things fall, that a person cannot be in two places at once, that Paris is a city and a city is a kind of place. It ships as a formal ontology of hundreds of thousands of concepts, which Cyc calls collections and individuals, wired together by millions of assertions built from predicates and a deep hierarchy of types.

The last public release landed in 2012 as downloadable OWL files. You load those into a triple store or an RDF library and query them much as you would any other knowledge graph. There is no live API and no ongoing maintenance, so treat it as a fixed snapshot rather than a service. Because cyc.com is now Cycorp's commercial site, the original OpenCyc downloads can take some tracking down, but archived copies remain available.

For RAG, its value is as a scaffold of concepts and types rather than a well of facts to retrieve. It earns its keep if you want to tag entities against a principled category hierarchy, add common-sense typing to your pipeline, or study how hand-built symbolic knowledge is organised.

The caveats are real. It is frozen, so nothing reflects the world after 2012, and its instance data, meaning actual named things, is thin next to a live source like Wikidata. It is also only a slice of full Cyc: the inference engine and the richest rules stay proprietary. The redeeming feature is the licence. Apache 2.0 is genuinely permissive, so commercial use is fine with no ShareAlike or attribution strings attached.

Reach for Wikidata or DBpedia when you need broad, current, well-populated facts, and ConceptNet for lighter common-sense links. OpenCyc complements those with formal structure rather than scale.