Cyc Knowledge Base
Long-term AI project (since 1984) with ~1.5M terms and ~24.5M assertions of common sense knowledge.
Visit WebsiteBrowse the tree, filter it, or click any class to see just its corner of it.
Where this one tends to earn its keep.
Common sense reasoning for AI systems
Expert system knowledge backends
Natural language understanding
Automated reasoning and inference
I haven't checked this label against the actual licence text — it's the catalogue's original guess. Check the ontology's own site before you rely on it.
Others worth a look in Foundational.