Access X selection from command line

http://www.vergenet.net/~conrad/software/xsel/

User preference utility for X

Root window parameter setting utility for X

X Session Manager

Mono based web server

https://www.mono-project.com/

Utility for monitoring keystrokes on remote X servers

http://www.acm.vt.edu/~jmaxwell/programs/xspy/xspy.html

Watches the growth of files or directories

http://www.unicom.com/sw/xtail/

Multi-dimensional arrays with broadcasting and lazy computing

https://github.com/xtensor-stack/xtensor

Terminal emulator for the X Window System

https://invisible-island.net/xterm/

Outputs X window titles

https://github.com/baskerville/xtitle

Template library that supports other quantstack projects

https://github.com/xtensor-stack/xtl

Abstract network code for X

X11 program that displays images of various formats

http://www.trilon.com/xv/

Opensource MPEG-4 codec, based on OpenDivx

http://www.xvid.org/

Video mode tuner for X

Fast and functional image viewer

https://github.com/linuxmint/xviewer

Print out X-Video extension adaptor information

X Clients under Wayland

https://wayland.freedesktop.org/xserver.html

X11 server as Wayland client (development snapshot)

https://wayland.freedesktop.org/xserver.html

Dump an image of an X window

Window information utility for X

Image displayer for X

Extremely fast non-cryptographic hash algorithm

https://cyan4973.github.io/xxHash/

XKB keyboard layout indicator and switcher

https://sourceforge.net/projects/xxkb/

Magnify, rotate, mirror the image on the X screen

Yet another Basic for Unix and Windows

http://www.yabasic.de/

Display GNOME dialogs from the command line

https://github.com/v1cont/yad

Yet another FTP client, similar to ftp(1)

http://www.yafc-ftp.com/

Yet Another File Integrity Checker, similar to Tripwire

http://philosophysw.com/software/yafic/

Portable JSON parsing and serialization library in ANSI C

http://lloyd.github.com/yajl/

Enhanced execution box with regex pattern matching

http://offog.org/code/

Default GTK theme for Ubuntu

https://github.com/ubuntu/yaru

Default icon theme for Ubuntu

https://github.com/ubuntu/yaru

Complete rewrite of the NASM assembler

https://yasm.tortall.net/

General-purpose console screen reader

http://yasr.sourceforge.net/

Z39.50/SR client and API library

https://www.indexdata.com/resources/software/yaz/

Diff readability enhancher for color terminals

https://github.com/ymattw/ydiff

Terminal wrapper for creating drop-down consoles

http://phrat.de/yeahtools.html

Help browser for the GNOME 3 desktop

https://www.gnome.org/

Utilities to help manage documentation for Yelp and the web

https://wiki.gnome.org/Apps/Yelp/Tools

DocBook XSLT stylesheets for yelp

https://www.gnome.org/

Free (GPL) encoder and decoder for the yEnc Usenet file format

http://www.yencode.org/

Yubico C client library

https://developers.yubico.com/yubico-c-client/

Library and tool for personalization of Yubico's YubiKey

https://developers.yubico.com/yubikey-personalization/

Unpack data in MS Outlook TNEF format

https://github.com/Yeraze/ytnef

Yubico PIV tool

https://developers.yubico.com/yubico-piv-tool/

Seamless ssh-agent for YubiKeys

https://filippo.io/yubikey-agent

The Yahoo! JavaScript and CSS Compressor

https://yui.github.io/yuicompressor/

Tool to decode yEnc encoded files

http://yydecode.sf.net/

ZBar barcode reader

http://zbar.sourceforge.net