DocBook
DocBook is a very popular XML (and formerly SGML) vocabulary for writing technical documentation (among other things). Years ago, I collected DocBook resources here, but they've long since moved farther afield.
There are four principal sources for DocBook:
OASIS, the official home page for the DocBook Technical Committee, the maintenance organization for DocBook.
DocBook.org, the main website, and tdg.DocBook.org, official home of DocBook: The Definitive Guide , published by O'Reilly & Associates.
The DocBook organization at GitHub, for the DocBook XSLT stylesheets, a number of derived doctypes, and some older technologies like DSSSL.
Projects Not (Yet) Moved Elsewhere
A couple of now moribund efforts remain here.
A graphical representation of the flow of information through a complete XML publishing system for DocBook.
DocBook IDE
I've long since abandoned this project.