Lots of CMS that is free and open source. But not many XML-based CMS, I have a list of CMS is free and open source XML-based :
1. GetSimple is an XML based lite Content Management System. They have loaded it with features that every website needs, but with nothing it doesn’t. GetSimple is truly the simplest way to manage a small-business website.
2. Mapix CMS is a very simple CMS that allows to modify any content in your websites without using a database.
Mapix is a CMS/framework designed to develop websites entirely (as far as possible) with the XML/XSL/XSD technology. The objective of Mapix is to become a complete gateway between the HTTP server and XML services.
3. OpenCms from Alkacon Software is a professional, easy to use website content management system. OpenCms helps content managers worldwide to create and maintain beautiful websites fast and efficiently.
4. WebJaxe is a free CMS, to create websites with a web user interface. It works with PHP on server side, and with a Java applet on client side to edit the contents. The editor is based on Jaxe, an XML editor working as a desktop application.
5. SoftXMLCMS is a unique content management system for managing data in XML format. Easy graphical interface enables you to control the profiling data for the creation of hierarchical structures.
SAPID is a tool for site creation and management. SAPID is distinguished through the comfortable Ajax-based site structure management interface and high flexibility of design, content and functionality integration. SAPID does not need DB (file-flat CMS).
sharpcms sharpcms is a open source content management system based on XML and XSLT and written in C#.
Kumera is an Open Source Content Management System written in Perl and using XML for data storage, designed for small to medium web sites. You can read more about what it does in features, or download it and try it for yourself.
Apache Lenya is an XML CMS based on Cocoon. The session will cover the use of Lenya as a CMS that works with XML. Features such as revisioning, publication templating, binary data, and the workflow engine will be covered. Of particular interest will be how Lenya handles arbitrary XML content. The session will cover how to store XML either by using a form to create the document, or creating and editing content via one of the available editors. Finally, the session will cover how to transform content in the system into XHTML or other forms of XML.