Program to link multiple X servers together

https://github.com/dottedmag/x2x

Two-pass cross-assembler for MOS 6502 and compatible CPUs

http://www.floodgap.com/retrotech/xa/

Apache XSLT processor for transforming XML documents

http://xml.apache.org/xalan-j/

Play most popular animation formats and show pictures

http://smurfland.cit.buffalo.edu/xanim/home.html

Cross-desktop libraries and common resources

https://github.com/linuxmint/xapp

OpenDarwin XML eXtensible ARchiver

http://mackyle.github.io/xar/

X authority file utility

Program to adjust backlight brightness

Motif-based widget which displays a grid of cells as a spreadsheet

http://xbae.sourceforge.net/

Mailbox flag for X

X.Org bitmaps data

Brightness and gamma correction through the X server

http://disjunkt.com/xbrightness/

Scientific calculator for X

Tool for managing x11 cut-buffers

http://oldhome.schmorp.de/marc/xcb.html

The X protocol C-language Binding (XCB) protocol

https://xcb.freedesktop.org/

Module with libxcb/libX11 extension/replacement libraries

https://xcb.freedesktop.org/

XCB cursor library

https://xcb.freedesktop.org/

Pretty-print XCB codes

https://xcb.freedesktop.org/

Port of Xlib's XImage and XShmImage functions

http://xcb.freedesktop.org/

Standard X key constants and conversion to/from keycodes

http://xcb.freedesktop.org/

Convenience functions for the Render extension

https://xcb.freedesktop.org/

Framework for window manager implementation

http://xcb.freedesktop.org/

XCB utility functions for the X resource manager

https://github.com/Airblader/xcb-util-xrm

X11 IRC client using the GTK+ 2 toolkit

http://xchat.org/

Interface to X selections ("the clipboard") from the command line

https://github.com/astrand/xclip

X clipboard client

Analog and digital clock for X

http://www.freedesktop.org/Software/xapps

Device Color Characterization utility for X

Sample X compositing manager

http://www.freedesktop.org/Software/xapps

Monitor system console messages with X

X.org cursors themes

Create an X cursor file from a collection of PNG images

Sets the background of your X desktop under water

http://xdesktopwaves.sourceforge.net/

Tools to allow all applications to integrate with the free desktop

https://www.freedesktop.org/wiki/Software/xdg-utils/

Display ditroff output

X.Org X display manager

Programmatically simulate keyboard input or mouse activity

https://www.semicomplete.com/projects/xdotool/

Display information utility for X

Query configuration information of DRI drivers

Simple xargs and apply replacement

https://github.com/leahneukirchen/xe/

Set the root window to the image of earth

https://xearth.org/

Small but powerful graphical text editor

https://github.com/linuxmint/xed

Simple text editor for X

X server from X.Org based on kdrive

https://www.x.org/

Validating XML parser from the Apache XML Project

https://xerces.apache.org/xerces-c/

XML parser for Java

http://xerces.apache.org/xerces2-j/

Print contents of X events

Follow the mouse X demo

X.Org elographics input driver

https://www.x.org/

X.Org event device input driver

https://www.x.org/