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
XML Sapiens - A descriptive language for user interfaces of CMS-based web sites. - http://xmlsapiens.org
Active Tags - A language for XML native programming, where usual procedure-oriented sentences can be mixed with declarative sentences. - http://disc.inria.fr/perso/philippe.poulard/xml/active-tags/
CDuce - A typed, functional programming language adapted to the manipulation of XML documents. - http://www.cduce.org/
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-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