XML - eXtended Markup Language

Altova: XML Spy
Codetoad
Editix
Ephox
HotScripts
Liquid XML: free XML / XSD editor
MoreOver: news from 1500 online sources in XML format (to be used in websites or applications)
Open XML Community
Open XML Developer
Oxygen XML Editor
Serence: Klip provider
Tout est facile: PHP, XML, SQL (FR)
VoiceXML
W3Schools: XML
W3Schools: XSLT Reference
WebServices Summit
XML in one page
XML.com
XML.org: een bibliotheek van XML termen
XML Files
XMLfr
xmlpitstop
XML News
XPL

The eXtensible Presentation Language (XPL) has been developed by Engineering Ingegneria Informatica S.p.A.within national research projects with the intention to make easier the presentation of contents supplied by Web-based services in thematic networks ontologically described. eXtensible Presentation Language is a language based on a methodological approach that supports designers in addressing issues related to the user interfaces requirements both for the design and the development phases.

Altova
CryptoGateway X-Script
EditiX XML Editor
Peter's XML editor
Stylus Studio
XQuery
Zorba is a general-purpose XQuery processor implementing in C++ the W3C family of specifications. It is not an XML database. The query processor has been designed to be embeddable in a variety of environments, such as other programming languages extended with XML processing capabilities, browsers, database servers, XML message dispatchers, or smartphones. Its architecture employes a modular design, which allows customizing the Zorba query processor to the environment’s needs. In particular, the architecture of the query processor allows a pluggable XML store (e.g. main memory, DOM stores, persistent disk-based large stores, and S3 stores).