Libxml2 is the XML C parser and toolkit developed for the Gnome project (but usable outside of the Gnome platform).
libxml2
Programming
xml
parser
C
reference
xml-parser
License:MIT
DocBook is an XML / SGML vocabulary that enables you to create document content in a presentation-neutral form that captures the logical structure of the content.
Open source stylesheets also exist that allow you to transform DocBook content into a variety of formats.
docbook
Documentation
publishing
help
xsl-fo
html
pdf
XSLT
xml
License:MIT
XMLStarlet is a set of command line XML utilities which can be used to transform, query, validate, and edit XML documents and files using a simple set of shell commands, similar to the way it is done for plain text files using grep/sed/awk/tr/diff/patch.