The web site http://www.xml.com is one of the most complete and timely sources of XML information and news around. It should be on your weekly reading list if you are learning or using XML.
Sponsored by OASIS, http://www.xml.org has XML news and resources, including the XML Catalog, a guide to XML products and services.
Edited by Robin Cover, http://xml.coverpages.org/ is one of the largest and most up-to-date lists of XML resources.
Elliotte Rusty Harold provides almost daily news, along with a quote of the day, at http://ibiblio.org/xml.
For programmers itching to work with XML, http://www.xmlhack.com is a good place to go for news on the latest developments in specifications and tools.
OASIS, the maintainers of DocBook, have a web page devoted to the XML application at http://www.docbook.org/. You can find the latest version and plenty of documentation here.
Jukka Korpela has assembled a huge amount of information related to character sets at http://www.cs.tut.fi/%7Ejkorpela/. The tutorial is well written and very interesting reading.
Signing up with the XSL mailing list is a great way to keep up with the latest developments in XSL and XSLT tools and techniques. It's also a forum for asking questions and getting advice. The traffic is fairly high, so you should balance your needs with the high volume of messages that will be passing through your mailbox. To sign up, go to http://www.mulberrytech.com/xsl/ and follow the instructions.
The xml-dev mailing list is an extremely busy forum for discussion of all kinds of XML-related issues and best practices. To sign up, go to http://lists.xml.org/ and follow the instructions.
This part of the Apache project focuses on XML technologies and can be found at http://xml.apache.org. It develops tools and technologies for using XML with Apache and provides feedback to standards organizations about XML implementations.
This guide is the Microsoft Developers Network's online workshop for XML. It contains information about using XML with Microsoft applications and can be found at http://msdn.microsoft.com/xml/XMLGuide/.
This journal contains articles, resources, opinions, news, and reviews covering all aspects of programming. Go to http://www.ddj.com for online content, and subscribe to the magazine while you're there.
Perl is an interpreted programming language for any kind of text processing, including XML. The best place online for information or to download code and modules is http://www.perl.com.
The best source for Java news and information is http://www.javasoft.com. Java is a programming language available for most computers and contains a lot of XML support, including implementations of SAX and DOM, as well as several great parsers.