A package collecting the tools required for writing basic XML applications in Python, along with documentation and sample code. Features include (but are not limited to) SAX, DOM, the xmlproc validating parser, and an Expat interface.

A couple of online utilities for converting XML and DTDs to XSL.

Tool to debug XSL transformations which is part of Stylus Studio. [Commercial]

Xalan is an XSLT (Extensible Stylesheet Language Transformation) processor for transforming XML documents into HTML, text, or other XML document types. Has various versions (C++, Java) available. (Was initially called LotusXSL by IBM Alphaworks)

A text based tool to debug stylesheets and it is similar to the Unix/Linux debugger gdb. It uses using the libxslt library.

A powerful editor with XSLT processing and debugging capabilities. You can run an XSLT processor on the Emacs buffer you edit, and view the results either in another Emacs buffer or in a browser.

The XSLTDoc Application helps you to browse and understand XSLT stylesheets. It shows summaries of stylesheets and explains each XSLT instruction in detail. This page also has several other useful utilities.