Robin Cover's OASIS SGML/XML Web Page
The authoritative place to start to learn about all things XML.

The W3C XML Activity
This is the organization that defined XML and related specifications.

XML.org
The vendor-neutral clearinghouse for XML resources and information about using XML in industry.

XML.com
Articles about and reviews of XML technology and products.

Open Directory XML information
Search result on "XML" from within the Open Directory project web information hierarchy.