The SGML/XML Web Page
Database containing over 2000 documents pertaining to SGML, XML, and related standards. Features a bibliography for SGML, vendors and service providers, software reviews, mailing lists, user groups, and projects.
Charles F. Goldfarb's SGML Source Home Page
Includes the InFrequently Asked Questions.
Comparison of SGML and XML
A Note from the W3C detailing the differences and similarities between the two markup languages.
Overview of SGML Resources
SGML links at the W3C, meant for use in reviews of the HTML specification.
SGML/XML character entity reference with LaTeX equivalents
This is an attempt to make an overview of SGML/XML character entities in ISO 8879:1986 and their LaTeX equivalents. It is also a handy reference to the ISO character entities for non-LaTeX users as it shows the visual appearance of the character entities.
Formerly Linuxdoc-SGML, SGMLtools offers a out-of-the-box SGML processing solution for technical (DocBook) documentation. [Open source, GPL]