Free Web Services Resource Sites
MSDN Web Services Developer Center
Microsoft's web services resource site.
SOA and web services
This is IBM's resource center for web services technology. The site features tutorials, articles and many other web services resources, some of which may require registration (pretty sure it's free though).
Web Services Tutorial: Understanding XML and XML Schema
This Web Service article series covers all the important standards in the Web Services stack and ties them together with real-world examples. The first article in this series discusses XML (Extended Markup Language).