xTests is a small, simple, lightweight, portable unit/component testing library for exercising C and C++ libraries. It relies on no platform-specific or compiler-specific constructs and doesn't require pre-processing of your source code by scripting languages or use macros to create secret classes that use Schwarz counters to register test cases. It relies on you to simply code what you want, and nothing that you don't want.
XWidglets is a complete rich Java Swing tool for designing, creating, and using XML-based GUIs. It provides a lightweight XML client, clear separation between view, data, and process, MVC I and II support, event management, a consistent look and feel across platforms, a rich set of GUI components, and n-tier architecture integration.
xcb-proto contains descriptions of the X Window System core protocol and selected extensions in an XML-based data format. They are used by the X C Binding (XCB) library to generate much of its code, but you can also use these descriptions for other purposes, such as decoding the X protocol or building X Window System bindings for other languages.
The Xitnalta Content Management System (xcms) provides a virtual filesystem library to deal with content from different sources (a real filesystem, a database, etc.) and in different formats. Its goal is to provide yet another way to access and manage data both locally and over the Internet.
xdelimitarea is an X Windows program that provides a non-intrusive way of finding out the location (with respect to the root window) and dimensions of an arbitrary rectangular area on a computer screen. This tool might be helpful for GUI or Web designers who are concerned with layout issues.