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.

Liquid Technologies XML Studio
Liquid XML Studio is an advanced XML editor and toolkit to simplify working with all aspects of XML technology.

OpenXMLDeveloper.org
Information and news about OpenXML formats.

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.