Programming

Active Tags
A language for XML native programming, where usual procedure-oriented sentences can be mixed with declarative sentences.
http://ns.inria.fr/active-tags/

CDuce
A typed, functional programming language adapted to the manipulation of XML documents.
http://www.cduce.org/

MSDN: XML in Office Developer Portal
Microsoft site offering articles, SDKs, reference documentation, downloads, and webcasts about working with XML in MS Office.
http://msdn2.microsoft.com/en-us/office/aa905545.aspx

SAX
Official website for SAX, the Simple API for XML, project resources include archived mailing lists, SAX evolution and a download area.
http://sax.sourceforge.net/

XML Sapiens
A descriptive language for user interfaces of CMS-based web sites.
http://xmlsapiens.org

XML-RPC
A specification and a set of implementations that allow software running on disparate operating systems, running in different environments to make procedure calls via HTTP and XML.
http://www.xmlrpc.com