DecisionSoft - Pathan - XPath compliant parsing and evaluation module for the Xerces-C, allows arbitrary selection of nodes from an XML document.
DOM Level 3 XPath Specification - Provides simple functionalities to access a DOM tree using XPath 1.0. This module builds on top of the Document Object Model Level 3 Core. [W3C Working Draft 31 October 2001]
Schematron - A tree-pattern schema language built on top of XSL with XPath.
Stylus Studio: XPath Tools - Tools for editing, generating and testing XPath 1.0 and XPath 2.0 expressions.
W3 Schools XPath - Tutorial which covers syntax, locations, expressions, functions and examples.
XML Path Language (XPath) Version 1.0 - Official W3C site describing this language for addressing parts of an XML document which is designed to be used by both XSLT and XPointer. (W3C Recommendation 16 November 1999)
XPath Package for OCaml - This package implements the full W3C XPath recommendation for the OCaml language; it may work with the output of PXP (XML parser), or with any implementation of the XPath data model.
Zvon: XPath Tutorial - A tutorial with several examples and pictures which can be downloaded for off-line use.
