W3C DOM

Apache DOM Implementation in Java
A validating XML parser written in 100% pure Java, which includes an implementation of the Level 1 DOM, some of the Level 2 DOM, and SAX.
http://xerces.apache.org/xerces2-j/

Apache's DOM Implementation in C++
XML for C++ parser (XML4C) is a validating XML parser written in a portable subset of C++. It implements Level 1 and some of Level 2 DOM, as well as providing SAX interfaces. Source code, samples and API documentation are provided with the parser.
http://xml.apache.org/xerces-c/

David Baron's DOM Tests
Tests in JavaScript for compliance with some parts of the W3C DOM specs.
http://dbaron.org/dom/test/

DocZilla - an SGML/XML browser
Built on Mozilla code, this browser has the same support for the DOM as Mozilla does.
http://www.doczilla.com/index.html

4DOM
A full DOM level 2 implementation for Python, included in the PyXML package.
http://pyxml.sourceforge.net/topics/

DOM 1 Reference
Reference guide with examples.
http://zvon.org/xxl/DOM1reference/Output/index.html

DOM2 Reference
Reference guide with examples for the second release of the object model.
http://zvon.org/xxl/DOM2reference/Output/index.html

DOMJuan: Koala's implementation of the DOM
Java implementation of the DOM Level 2. Free and with source code. Currently (version 0.1) only the Core and Events modules are available.
http://koala.ilog.fr/domjuan/

The Gnome DOM Engine
Promises to create a fully standards-compliant DOM interface. It is based on Daniel Veillard's libxml.
http://www.levien.com/gnome/gdome.html

Microsoft Internet Explorer - XML/HTML Browser
Full support for the Level 1 DOM.
http://www.microsoft.com/windows/ie/default.htm

Mozilla - XML/HTML browser
An open-source XML/HTML browser which is implementing full Level 2 DOM support.
http://www.mozilla.org/

Oracle XML Developer's Kit
Contains a parser with a Level 1 DOM interface. Available in Java, C, C++ and PL/SQL.
http://www.oracle.com/technology/xml/

Ozone
Open source, Java, OO DBMS which includes a DOM interface.
http://www.ozone-db.org/

Parsed XML Project
Project implementing the DOM as a persistent object in the Zope application server. Currently implements Core levels 1 and 2, XML level 2, and some Core and Loading level 3.
http://wiki.zope.org/ParsedXML/

Robin Cover's DOM Pages
A list of DOM resources.
http://xml.coverpages.org/dom.html

Unicorn XML Processor
A stand-alone ECMAScript interpreter that supports Level 1 DOM Core. An XSLT processor contains the same engine.
http://www.unicorn-enterprises.com/

xmlBlaster
XML and CORBA/RMI/XML-RPC based MOM (message oriented middleware) for Java/C/C++/Perl/Tcl/Python. Free for commercial and non commercial use (LGPL).
http://www.xmlblaster.org/

XML4P Perl DOM
Includes a collection of Perl5 wrapper objects that internally use their XML4C counterparts for Level 1 DOM processing.
http://xerces.apache.org/xerces-p/