A list of XML resources:

http://www.w3.org/XML/

http://www.xml.com/

http://www.xml.org/xml/registry.jsp

http://www.xml.org/

XML FAQ:
http://www.ucc.ie/xml/

XML Namespace FAQ:
http://www.rpbourret.com/xml/NamespacesFAQ.htm

http://www.perfectxml.com/

http://www.syncml.org/

http://www.newsml.org/

http://www.relml.org/

http://www.ecx-xml.org/

http://www.catxml.org/

http://www.openapplications.org/

http://www.xcbl.org/

http://www.ontology.org/

http://www.bpmi.org/

http://www.cxml.org/

Universal Business Language (UBL):
http://www.oasis-open.org/committees/ubl/

http://www.opentravel.com/

Global insurance standards:
http://www.acord.org/

http://www.oasis-open.org/

http://www.ebxml.org/

http://www.rosettanet.org/

http://www.biztalk.org/

http://www.xmlrpc.com/

http://www.uddi.org/

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

http://www.w3.org/2000/xp/

http://www.saxproject.org/

http://www.jdom.org/

http://www.dom4j.org/

http://www.rpbourret.com/xml/XMLDataBinding.htm

http://www.rpbourret.com/xml/SchemaMap.htm

http://www.rpbourret.com/xml/XMLAndDatabases.htm

http://www.rpbourret.com/xml/XMLDatabaseProds.htm

http://www.rpbourret.com/xml/XMLDBLinks.htm

http://www.25hoursaday.com/StoringAndQueryingXML.html

http://www.dbxml.org/

http://www.xmldb.org/

http://www.rpbourret.com/xmldbms/index.htm

http://www.xmldatabases.org/

http://www.sqlx.org/

eXcelon - XML datastore vendor:
http://www.exceloncorp.com/

http://xml.apache.org/

http://xml.apache.org/soap/

Apache Axis (an implementation of SOAP):
http://xml.apache.org/axis/

JBoss SOAP (aka "JBoss.net"):
http://www.jboss.org/developers/jboss-net.jsp

XHTML:
http://www.w3.org/TR/xhtml1/

http://java.sun.com/xml/

Extensible Media Commerce Language (XMCL):
http://www.xmcl.org/

Extensible Rights Markup Language (XRML):
http://www.xrml.org/ (related link: http://www.idrm.org/

Open Digital Rights Language (ODRL):
http://www.odrl.net/

http://groups.yahoo.com/group/wbxml

http://wbxml4j.sourceforge.net/

http://zeus.enhydra.org/

http://castor.exolab.org/

http://www.voicexml.org/

Open Financial Exchange:
http://www.ofx.net/

Financial Products Markup Language:
http://www.fpml.org/

http://www.ifxforum.org/

http://www.hr-xml.org/

MathML:
http://www.w3c.org/Math/

DARPA Agent Markup Language:
http://www.daml.org/

Sheet music:
http://www.musicxml.org/

Directory Services Markup Language:
http://www.dsml.org/

XML User-interface Language (XUL):
http://www.mozilla.org/xpfe/

XUL tutorial:
http://www.xulplanet.com/tutorials/xultu/

XML tutorial:
http://www.w3schools.com/xml/default.asp

XML Metadata Interchange (XMI):
http://www.omg.org/technology/documents/formal/xmi.htm

http://www.xmltrustcenter.org/

Verisign XML:
http://www.verisign.com/developer/xml/

Conference: http://www.xmlconference.org/

Conference: http://www.xmlconference.com/

Conference: http://www.xmlworld.org/

OpenGroup:
http://www.opengroup.org/

news:comp.text.xml

http://www.xml.gov/

CommerceNet:
http://www.commerce.net/

http://www.businessinternetconsortium.org/