NELL (Never-Ending Language Learner)
NELL runs as a continuous experiment: it reads web pages, proposes new facts about entities and how they relate, and uses what it already knows to judge and improve the next round of extractions. Over years of running, this has grown into a large store of candidate beliefs, each with a confidence score reflecting how sure the system is.
The knowledge base is published for download, so you can browse or reuse the extracted facts. Because the beliefs come from automatic reading rather than human curation, quality varies, and the confidence scores matter when you decide which facts to trust.
For RAG, NELL is mainly of research interest: a case study in learning structured knowledge from text at scale, and a source of web-derived facts if you are willing to filter by confidence.
Related sources
ConceptNet
A multilingual common-sense knowledge graph that links words and phrases with labelled connections, such as "a cat is a pet" or "rain causes wet". It captures the everyday relationships between ideas that plain text rarely spells out.
Google Knowledge Graph API
An API into Google's Knowledge Graph, the store of billions of facts about entities that powers the info panels you see beside search results. You send a name or query and get back matching entities with descriptions, types, and links. Free to use within rate limits.
WordNet
A lexical database of English that groups nouns, verbs, adjectives, and adverbs into sets of synonyms called synsets, then links those sets by meaning. It maps how words relate, which sense means what, what is a kind of what, so software can work with meaning rather than just spelling.