Research Data Alliance Metadata Directory - The RDA Metadata Directory is a collaborative, open directory of metadata standards applicable to scientific data. Subject areas include arts and humanities, engineering, life sciences, physical sciences & mathematics, social & behavioral sciences, and general research data (multidisciplinary).
Linked Open Vocabularies (LOV) - LOV provides a searchable repository of vocabularies and ontologies used to describe many different disciplines and domains.
Data Documentation Initiative - DDI is an international standard for describing statistical and social science data. It contains a metadata specification, as well as a list of tools to help researchers work with DDI metadata.
FAIRsharing - FAIRsharing (formerly BioSharing) offers a searchable database of metadata standards, markup languages, taxonomies, and other resources for all disciplines.
BioPortal - BioPortal offers an extensive repository of biomedical ontologies, including a recommender tool to help choose the best ontology for your research.
Open Metadata Registry - The Metadata Registry provides services to developers and consumers of controlled vocabularies and is one of the first production deployments of the RDF-based Semantic Web Community's Simple Knowledge Organization System (SKOS).