RDF Schema
Basic elements for describing ontologies — classes, subclass hierarchies, properties, domains, and ranges using RDF.
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.
Define lightweight class hierarchies
Specify property domains and ranges
Create simple vocabulary structures
Foundation for OWL ontologies
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 Web Standards.