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The XML FAQ — Frequently-Asked Questions about the Extensible Markup Language
Section 2: Existing users
Q 2.1: What do I have to do to use XML?
To read it: use any modern web browser; to create it: use an XML editor.
For the average user of the Web, you don't need anything except a browser which works with XML (see the question about browsers). Remember new XML-related facilities are being invented or implemented all the time (see the W3C web site), so some recent features may not work in all browsers yet.
You can use XML-conformant browsers to look at some of the stable XML material, such as Jon Bosak's Shakespeare plays and the molecular experiments of the Chemical Markup Language (CML). There are some more example sources listed at http://xml.coverpages.org/xml.html#examples, and you will find XML (particularly in the guise of XHTML) being introduced in places where it won't break older browsers.
If you want to start preparations for creating your own XML files, see the questions in the Authors' Section and the Developers' Section, particularly the question on XML Editors.