For a good list of existing schemas, see SchemaWeb.info. Some of the more popular schemas are linked below:

Core vocabularies:

• RDF
  namespace: http://www.w3.org/1999/02/22-rdf-syntax-ns#
• RDFS: RDF Schema
  namespace: http://www.w3.org/2000/01/rdf-schema#
• OWL: Web Ontology Language
  namespace: http://www.w3.org/2002/07/owl#
• Dublic Core (DC)
  namespace: http://purl.org/dc/elements/1.1/

Other Vocabularies:

• BIO: A vocabulary for biographical information.
  namespace: http://purl.org/vocab/bio/0.1/
• DOAP: Description of a (open-source) Project
  namespace: http://usefulinc.com/ns/doap#
• FOAF: Friend of a Friend
  namespace: http://xmlns.com/foaf/0.1/