The Cover Pages
The Cover Pages is a reference collection supporting the SGML/XML family of markup language standards and their application. The principal objective in this public access knowledgebase is to promote and enable the use of open, interoperable standards-based solutions which protect digital information and enhance the integrity of communication. A secondary objective in The Cover Pages is to provide reference material on enabling technologies compatible with descriptive markup language standards and applications: object modeling, semantic nets, ontologies, authority lists, document production systems, and conceptual modeling. It is hosted by OASIS.
More information: The Cover Pages website.
More on the general topic: XML Resources