Microformats
- Simple data formats built upon existing and widely adopted standards. Overview, specifications, events, and discussion.
- http://microformats.org/

Electronic Documents Markup
- A brief explanation of what markup languages are and where they come from.
- http://web.cs.wpi.edu/~kal/elecdoc/EDmarkup.html

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.
- http://yaml.org/

whatis.com: markup
- A short definition.
- http://whatis.techtarget.com/definition/0,289893,sid9_gci212527,00.html

Project Xanadu
- The original hypertext project. Includes a history of the project and gives credit to the early developers of markup languages.
- http://www.xanadu.com/