MDC-XSL
XSLT processor written in C++ [Open source, GPL]

XSL Trace
A tool that allows stepping through XSL scripts showing the transformation rules as they are created and the XML or HTML as it is generated.

XML::XSL
A server side XSLT processor written in Perl [Open source, GPL].

Unicorn XSLT Processor
XSLT processor for Windows written in C++ [Freeware].

XSplit
Takes standard HTML file and splits it automatically into two related XML files. The first file contains the content definition (DTD), the second contains the format definitions for that page. Once split, the static formatting (XSL) can be applied to new XML content files.

libxslt
XSLT C library developped for the Gnome project [Open Source, LGPL].

XSLScript
Terse notation for writing complex XSLT stylesheets

XSL Processor for ColdFusion
CFX Tag allows you to perform XSL Transformations with ColdFusion. Free developer edition, commercial sites must purchase a license per server.

XSet
An XML property set description of XML 1.0 and XML namespaces that enables XPath based indexing and addressing of the full fidelity grove of an XML document.

XSL Debugger
Tool to debug XSL transformations. [Freeware]

Xalan-C++
XSLT Processor written in C++. A part of Apache's XML project [Open Source].

PassiveTeX
A library of TeX macros which can be used to process an XML document which results from an XSL transformation to FO (formatting objects). [LaTex]

XSL stylesheets for TEI XML
A set of XSLT specifications to transform Text Encoding Initiative (TEI) XML documents to HTML, and to XSL Formatting Objects (XSL-FO).

XML/XSL Portal
This site provides a significant number of utilities and downloads for XSLT and related developers. Quite advanced in its own use of client-side XML it's unfortunately not available to non-IE5+ browsers.

The SAXON XSLT Processor
Java XSLT processor developed by Michael Kay [Open source, MPL].

4XSLT
A component of the 4suite collection. A XSLT Processor written in Python by FourThought LLC [Open Source].

Microsoft XML Core Services 4.0
Contains an XSLT engine, which fully implements the XSLT standard [Freeware].

ShoXS: a Shorter XSL Syntax
ShoXS is a Shorter Syntax for XSL and this site provides some tools to read XSL files, convert and edit in ShoXS then convert back and save in XSL [Open Source, GPL].

Scriptura
A letter, report, and output generator for XSLT/XSLFO documents. GUI Designer for stylesheets, plus an Engine to create XSLT, XSLFO, PDF, XHTML, and PCL5. [Commercial].

DataPower Products
DGXT is an optimized software-based XSLT interpreter that is cross-platform and fully compliant with the current XSL specification. The XA35 XML Accelerator is a hardware-based solution that speeds XSLT, XPath, XSD (Schema validation) by 30X or more [Commercial].

Sablotron
Sablotron is an attempt to develop fast, compact and portable XSLT processor. Supported platforms: Linux, Windows NT and Solaris [Open Source, GPL].

XSL Transformations (XSLT) in Mozilla
Information about Mozilla's native XSLT processor. Includes a FAQ and a list of known bugs.

xsl:easy
Transformation tool using built-in graphical XSLT editor to create XML out of arbitrary data and save the transformation result back to other formats [Commercial].

Ecrion XF Rendering Server
Uses XSL formatting objects to dynamically create customized PDF, POSTSCRIPT, HTML, GIF, JPEG, or PNG files from XML data [Commercial].

Xsltp.pl
Perl CGI XSLT processor [Open Source].

XSL Template Designer
Includes a Windows GUI that simplifies designing layouts for complex XML forms and a runtime Engine that merges a layout with XML data to produce a PDF via XSL-FO [Commercial].