IdRef is the authority database (author, title) of the union catalogue for France's higher education libraries (SUDOC)
The Virtual International Authority File (VIAF) is an international authority file. It is a joint project of several national libraries
GeoNames is a geographical database available and accessible through various web services, under a Creative Commons attribution license.
DBpedia is a project aiming to extract structured content from the information created as part of the Wikipedia project.