Open-source web browser and toolkit, on which are based other browsers like Netscape or Opera. It includes --among other things-- an XML parser and other XML extras, like a MathML implementation, and an XSLT processor that all comply with the W3C recommandations - unlike IE 6.
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Development environment. currently only support Java. The C++ plug-in is announced to be released soon.
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So many things to say...
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The Java API for XML Binding...
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The pack contains the following technologies: the Java API for XML Messaging (JAXM)version 1.0.1 EA1; the Java API for XML Processing (JAXP) version 1.2 EA1; the Java API for XML Registries (JAXR) version 1.0 EA1; Java API for XML-base RPC (JAX-RPC) version 1.0 EA1. Each of these APIs is also available separately. Notice, however, that the "Java for XML Tutorial" runs with the Java-XML Pack.
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As Apache describes it very well:

Xalan-Java is an XSLT processor for transforming XML documents into HTML, text, or other XML document types. It implements the W3C Recommendations for XSL Transformations (XSLT) and the XML Path Language (XPath). It can be used from the command line, in an applet or a servlet, or as a module in other program.

You need the Java Development Kit or Java Runtime 1.2.2, or 1.3.x, which you can obtain from "http://www.ibm.com/java/jdk or http://www.java.sun.com.

free Office suite compatible with Microsoft documents. It includes word processor, table spreadsheet, database, presentation (PowerPoint-fashion), documents binder, mail manager, chat interface, etc.

Jo! is a light-weighted servlet container, i.e. a web server that can deal with Java servlets.
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The servlet container that is used in the official Reference Implementation for the Java Servlet and JavaServer Pages technologies. Tomcat is part of the Jakarta project.
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Amaya is an nice XML editor developed by the W3C, with real-time validation, WYSIWYG interface, tree structure view and an interface to TIDY. It also includes a "collaborative annotation application based on Resource Description Framework (RDF), XLink, and XPointer".

Unfortunately, as far as I know, there's no XSLT processor associated.

Nice environment for developing web sites. The editor is highly customizable and supports and includes functionalities for most of the file types that are related to the web technologies, including scripting languages. You can instantly preview the result of your work, including the application of an XSLT stylesheet to an XML source, by simply accessing the 'Preview' tab.

You'll also find a bunch of helpful plugins on the web site, including validators --to valid your XML source against your DTD, and XSLT processors --the basic version of HTML-Kit doesn't include any XSLT tool. You also might want to install some extra tutorials or HELP files.

The basic version includes Tidy, which is the W3C tool that allows you to check, correct, convert and format your XML/HTML source files.

Development environment for XML, DTD, and XSLT documents. Enables you to choose the XSLT engine you want to use (MSXSL, xalan, and many others), and to test your XPATH expressions.
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