For open source software, check out The Apache XML Project at [ http://xml.apache.org ]. A good source of industry-specific DTDs or XML schemas is RosettaNet, at [ http://www.rosettanet.org ]. For other XML-related standards, go to the OASIS site at [ http://www.oasis-open.org ]. XML.org is a good source of XML industry updates and software, at [ http://www.xml.org ].
Companies with XML sites and resources include Sun, [ http://www.sun.com/xml; ] Microsoft, [ http://www.microsoft.com/xml; ] and IBM, [ http://www.ibm.com/xml ].
Finally, there is a wide array of general Oracle and XML educational sites. Some of our favorites are [ http://www.zvon.org ], [ http://asktom.oracle.com ],and [ http://www.xmlfiles.com ].