4Suite is a Python-based toolkit for XML and RDF application development. It features a library of integrated tools for XML processing, implementing open technologies such as DOM, RDF, XSLT, XInclude, XPointer, XLink, XPath, XUpdate, RELAX NG, and XML/SGML Catalogs. Layered upon this is an XML and RDF data repository and server, which supports multiple methods of data access, query, indexing, transformation, rich linking, and rule processing, and provides the data infrastructure of a full database system, including transactions, concurrency, access control, and management tools. It also supports HTTP, RPC, SOAP, and FTP, plus APIs in Python and XSLT.
Libxml2 is the XML C library developed for the Gnome project. The library code is portable (to Linux, Unix, Windows, embedded systems, etc.) and modular; most of the extensions can be compiled out. Libxml2 implements a number of existing standards related to markup languages, including the XML standard, Namespaces in XML, XML Base, Relax NG, RFC 2396, XPath, XPointer, HTML4, XInclude, SGML Catalogs, and XML Catalogs. In most cases, libxml tries to implement the specifications in a relatively strict way. To some extent, it provides support for the following specifications, but doesn't claim to implement them: DOM, FTP client, HTTP client, and SAX2. Support for W3C XML Schemas is in progress. It includes xmllint, a command line XML validator.
LyX is a document processor that encourages an approach to writing based on the structure of your documents, not their appearance. It is intended for people people who write and want their writing to look great without tinkering with formatting details, font attributes, or page boundaries. On screen, it looks like any word processor, but it uses the TeX engine for printed output and producing richly cross-referenced PDFs. It is stable and fully featured.
Sablotron is an XML toolkit which implements XSLT, DOM, and XPath. Sablotron is written in C++, and it can be used from C, Perl, Python, PHP, ObjectPascal, and via a command line interface. It supports the XSLT 1.0, XPath 1.0, and DOM Level 2 W3C specifications. It is designed to be as compact and portable as possible, and is maintained as an Open Source project by Ginger Alliance.
Libxslt is a C library for GNOME which allows developers to work with XSLT. It is based on libxml for XML parsing, tree manipulation, and XPath support. Also included is 'xsltproc', a command line XSLT processor. The library is written in plain C, making as few assumptions as possible, and sticking closely to ANSI C/POSIX for easy embedding. It should work on Linux, Unix, and Windows. Though not designed primarily with performances in mind, libxslt seems to be a relatively fast processor. It also include full support for the EXSLT set of extension functions as well as some common extensions present in other XSLT engines.
XIST is an extensible HTML and XML generator. It is also an XML parser with a very simple and Python-esque tree API. Every XML element type corresponds to a Python class, and these Python classes provide a conversion method to transform the XML tree (e.g. into HTML). XIST can be considered 'object-oriented XSLT'. XIST also includes a cross-platform templating language, Oracle utilities, and various other tools.
Txt2tags converts a text file with minimal markup to HTML, XHTML, SGML, DocBook, LaTeX, Lout, Man page, Creole, Wikipedia, Google Code Wiki, PmWiki, DokuWiki, MoinMoin, MagicPoint, PageMaker, AsciiDoc, or ASCII Art. It is simple and fast and features automatic TOC, macros, filters, include, tools, GUI, CLI, and Web interfaces, translations, and extensive documentation.