The XML FAQ

For the average user of the Web, nothing except use a browser which works with XML (see the question B.4, question about browsers). Remember some XML components are still being invented or implemented (see the W3C web site), so some features are still either undefined or have yet to be written.

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 question C.7, 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 section BAuthors' Section and the section BDevelopers' Section.