From single-source documentation to eBusiness applications, XML has become the standard which companies are using to tag their information. Here is a list of places around the web that are dedicated to the XML family of standards. These places will provide you with insight into how XML is being used around the world!

W3C
The World Wide Web Consortium is the keeper of the XML standards. Actually, it is the keeper of many other standards too, but if you're looking for information about the XML specification. Here is a great place to start.

OASIS
OASIS is a non-profit, international consortium that creates interoperable industry specifications based on public standards such as XML and SGML, as well as others that are related to structured information processing.

The DocBook Site
DocBook is the data type definition (DTD) used by most writing groups to format their documents. If you're thinking of using XML for technical documentation, you'll probably run across this standard very soon! The site is a bit of an advertisement for O'Reilly & Associates (sorry), but it does provide useful information on the DocBook DTD.

xml.com
Another O'Reilly site chocked full of news and information about XML as well as articles from the world's leading XML authorities.

Ken Holman's website
Ken is one of the world's leading experts in XML today. In the near future, we'll be offering online versions of Ken's XSLT and XSLFO courses. Ken has also recently published a book on XML. If you're the kind of person who likes standup training, we heartily recommend Ken's courses.