An extraordinarily powerful text editor, and so much more. Learn all about it at http://www.gnu.org/software/emacs/emacs.html.
An Emacs major mode for editing XML and SGML documents that is available at http://www.lysator.liu.se/~lenst/.
Information on SAX, the Simple API for XML, is available at http://www.saxproject.org. Here, you will find the Java source code and some helpful documentation.
A high-performance XSLT stylesheet processor that fully implements XSLT and XLinks. You can find out more about it at the Apache XML Project web site, http://xml.apache.org.
A fully validating parser that implements XML, DOM levels 1 and 2, and SAX2. Find out more about it at the Apache XML Project, http://xml.apache.org.
A Java implementation of XSLT, at http://www.jclark.com/xml/xt.html.