Altova - XML Tools
Probably the best suite of XML Editor, Data Management, UML and Web Services tools available.

Businessware Architects
Some useful XML and XML-schema tools and articles.

w3schools
Online full web building tutorials.

xFront - Tutorials and Articles on XML and Web Technologies.
Very useful tutorials and articles on XML and web technologies. Some complex and advanced topics are handled with ease.

XML From the Inside Out
O'Reilly Media Corporation website - XML technologies research & development.