Link: http://www.stylusstudio.com/xpath_editor.html
Source Author or Organization: Stylus Studio

The Stylus Studio XPath Editor provides real-time, XPath 1.0 and XPath 2.0-aware editing functionality, including XPath code-sensing, auto-completion of XPath expressions, XPath syntax highlighting, for XPath functions, axes, path expressions, and more. These features help XPath developers by prevent errors, and simplifying the XPath learning process.