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.
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.
itools is a collection of Python libraries which provides a wide range of capabilities, including an abstraction over directory and file resources, a search engine, type marshallers, datatype schemas, i18n support, URI handlers, a Web programming interface, a workflow interface, and support for data formats such as (X)HTML, XML, iCalendar, RSS 2.0, and XLIFF.
Gnosis Utils contains several Python modules for XML processing, plus other generally useful tools: xml.pickle (serializes objects to/from XML, API compatible with the standard pickle module), xml.objectify (turns arbitrary XML documents into Python objects), xml.validity (enforces XML validity constraints via DTD or Schema), xml.indexer (full text indexing/searching), and many more.
Rx4RDF is a set of technologies designed to make W3C's RDF less complex and easier to use. It includes RxPath, which enables one to use familar XML technologies like XPath, XSLT, and XUpdate to query, transform and update an RDF model, Raccoon, an application server that uses an RDF model for its data store and RxPath to translate arbitrary requests to RDF resources, and ZML and RxML, alternative text formats for XML and RDF that were designed to allow novices to write XML and RDF with (nearly) the same ease as a Wiki entry.
SimpleXML is a commandline application and library for very simply manipulating XML files by path to elements and regular expresions identifying data of elements. It lets you harvest data from XML files, change values of attributes, print or change data of elements, create new elements, etc.
WikklyText is a Python-based wiki and modular library for creating custom wikitext rendering applications. Its wikitext engine is compatible with the TiddlyWiki markup language. It functions equally well as a wiki-on-a-stick or as a multiuser wiki behind Apache. It includes a Drupal plugin as well.