Resources 

Triple-S utility software 

http://www.triple-s.org/ssssoftware

Free conversion software available for download. In some cases, more extensive software may be available from the authors.

Organisations 

http://www.w3.org/Consortium

W3C, the World Wide Web Consortium, was created to lead the World Wide Web to its full potential by developing common protocols that promote its evolution and ensure its interoperability. 

http://www.oasis-open.org

OASIS, the Organization for the Advancement of Structured Information Standards, is a nonprofit, international consortium dedicated to accelerating the adoption of product-independent formats based on public standards including SGML, XML and HTML. 

http://www.w3.org/TR/REC-xml

Extensible Markup Language (XML) 1.0.  

http://www.w3.org/TR/xslt

Extensible Stylesheet Language Transformations (XSLT) 1.0. 

http://www.xmlinfo.com

XMLINFO, along with its sister site XSLINFO, provides a well organised index of information about XML and XSL

http://www.xmlhack.com

The news site for XML developers which aims to distill essential news, opinions, tips and issues concerning XML and related development.

http://www.xmlmag.com

Online XML magazine with articles and features on XML.

http://www.egroups.com/group/xml-dev

XML-DEV is an informal unmoderated discussion list to support those who are interested in the implementation and development of XML.

http://www.brics.dk/~amoeller/XML/index.html

This 130+ page slide collection provides an introduction and overview of XML and related technologies, including selected links to more information about each topic 

Software resources

 http://www.xmlsoftware.com

XMLSOFTWARE, provides a well organised and frequently updated index of software resources on XML and XSL.

http://xml.apache.org

One of the goals of the Apache XML Project is to provide commercial-quality standards-based XML solutions that are developed in an open and cooperative fashion. Included in these are:-

Xerces - XML parsers in Java, C++ (with Perl and COM bindings) 

Xalan - XSLT stylesheet processors, in Java and C++

http://www.alphaworks.ibm.com

alphaWorks' mission is to provide early adopter developers direct access to IBM's emerging "alpha-code" technologies.

http://msdn.microsoft.com/xml

Microsoft Developer Network XML developer center. 

http://www.vbxml.com

Windows XML developers resource with an emphasis on Visual Basic and ASP.

http://www.xml.org

XML.ORG is hosted by OASIS and provides an independent resource for news, education, and information about the application of XML in industrial and commercial settings

http://www.schema.net

SCHEMA.NET, provides an index of published XML schemas