Electronic Documents Markup: A brief explanation of what markup languages are and where they come from.
Microformats: Simple data formats built upon existing and widely adopted standards. Overview, specifications, events, and discussion.
Project Xanadu: The original hypertext project. Includes a history of the project and gives credit to the early developers of markup languages.
whatis.com: markup: A short definition.
YAML Ain't Markup Language (YAML): A straightforward machine parsable data serialization format designed for human readability and interaction with scripting languages such as Perl and Python.