GUI toolkit (wxWidgets) with GTK+3 bindings

https://www.wxwidgets.org/

Modular FTP server configurable online using SITE commands

https://sourceforge.net/projects/wzdftpd/

X11 server performance test program

VNC server for use with real X displays

https://github.com/LibVNC/x11vnc

H.264/MPEG-4 AVC Video Encoding (Front End CLI)

https://www.videolan.org/x264.html

H.265/High Efficiency Video Coding (HEVC) format

https://www.x265.org/

Program to link multiple X and VNC servers together

https://fredrik.hubbe.net/x2vnc.html

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

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

X based alarm clock

https://ftp.gwdg.de/pub/x11/x.org/contrib/utilities/xalarm.README

Play most popular animation formats and show pictures

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

Bindings allowing Xapian to be used from various programming languages

https://xapian.org/

Probabilistic text search database engine

https://xapian.org/

Cross-desktop libraries and common resources

https://github.com/linuxmint/xapp

OpenDarwin XML eXtensible ARchiver

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

X authority file utility

TV viewing application and a few TV utilities

https://linuxtv.org/

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/

High-performance low-overhead BitTorrent tracker

https://xbtt.sourceforge.net/

Replacement for xbiff that handles multiple mail files

Scientific calculator for X

Calendar with a notebook for X11 (internationalized version)

Tool for managing x11 cut-buffers

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

Input method development support for XCB

https://github.com/fcitx/xcb-imdkit

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

https://xcb.freedesktop.org/

Standard X key constants and conversion to/from keycodes

https://xcb.freedesktop.org/

Convenience functions for the Render extension

https://xcb.freedesktop.org/

Framework for window manager implementation

https://xcb.freedesktop.org/

XCB utility functions for the X resource manager

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

X11 circuit schematics drawing program

http://opencircuitdesign.com/xcircuit/

Exciting Cite for any Mail/News reading modes on Emacs

https://www.gentei.org/~yuuji/software/

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

https://github.com/astrand/xclip

X clipboard client

Analog and digital clock for X

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

Device Color Characterization utility for X

Sample X compositing manager

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

MidnightBSD X configuration tool

https://github.com/midnightbsd/xconfig

Monitor system console messages with X

Multiwindow mouse-based text editor for X

http://xcoral.free.fr/

X.org cursors themes

Create an X cursor file from a collection of PNG images