docbook-util, xmlmind, XMLStarlet Command Line XML Toolkit (http://xmlstar.sourceforge.net/)
pandoc, ea sdf, xmlif ()
asciidoc, http://www.methods.co.nz/asciidoc/
xproc (http://xproc.org/). “XProc: An XML Pipeline Language”
lxml (http://lxml.de/). “lxml - XML and HTML with Python”. Especially the functionality for extending XSLT and XPath (http://lxml.de/extensions.html).