Alternate menu for the Xfce Desktop

https://goodies.xfce.org/projects/panel-plugins/xfce4-whiskermenu-plugin

Xfce's window manager

https://xfce.org/

Xfce 4 window decoration themes for xfwm4

http://www.xfce.org/

Keyboard layout switching plugin for the Xfce panel

https://goodies.xfce.org/projects/panel-plugins/xfce4-xkb-plugin

Compiles information about file transfers from logfiles

Utilities for managing XFS filesystems

http://oss.sgi.com/projects/xfs/

X11 multimedia player

http://www.xine-project.org

Front-end to an XSL toolchain

https://pagure.io/xmlto/

X Multimedia System -- audio player with Winamp GUI

http://www.xmms.org/

Pure Java napster client; also, supports OpenNap & giFT (FastTrack)

http://xnap.sourceforge.net/

Enhanced nedit with utf-8 support

https://sourceforge.net/projects/xnedit

Graphical performance meter

https://www.pogo.org.uk/~mark/xosview/

Display PDF files and convert them to other formats

https://www.xpdfreader.com/

Display PDF files and convert them to other formats

https://www.xpdfreader.com/

Display PDF files and convert them to other formats

https://www.xpdfreader.com/

Multi-format document reader

https://github.com/linuxmint/xreader

WindowMaker dockapp to control XScreenSaver

http://xscreensaverapp.sourceforge.net/

W3C XML Schema to C++ translator

https://www.codesynthesis.com/projects/xsd/

Tcl/Tk-based simple TeX interface

https://www.mat.univie.ac.at/~gerald/ftp/xtexsh/

Humorous Arcade game for X

https://xtux.sourceforge.net/

Opensource MPEG-4 codec, based on OpenDivx

http://www.xvid.org/

Fast and functional image viewer

https://github.com/linuxmint/xviewer

Run Xwayland and compositor headless

https://gitlab.freedesktop.org/ofourdan/xwayland-run

Sega Saturn system emulator

http://yabause.sourceforge.net/

WAD file editor, for games like Doom and Heretic

http://www.teaser.fr/~amajorel/yadex/

Yet another FTP client, similar to ftp(1)

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

Enhanced execution box with regex pattern matching

http://offog.org/code/

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/

Amateur Radio curses based logging program

https://fkurz.net/ham/yfklog.html

Improved version of Icculus Quake II

https://www.yamagi.org/quake2/

Command line ICQ client with ICQ2000 and recoding support

http://www.ysm.com.ar

Unpack data in MS Outlook TNEF format

https://github.com/Yeraze/ytnef

Multi-lingual Unicode text editor with TTF support

https://www.yudit.org/

Tool to decode yEnc encoded files

http://yydecode.sf.net/

Nibbles-like multiplayer game

https://zatacka.sourceforge.net/

Free RIPv1, RIPv2, OSPFv2, BGP4 route software (server/reflector)

http://www.zebra.org/

Event logging framework

https://wiki.gnome.org/Projects/Zeitgeist

Simpler web photo gallery

https://www.zenphoto.org/

User utilities for zisofs

C library for polynomial arithmetic

https://web.maths.unsw.edu.au/~davidharvey/code/zn_poly/

Intel x86 only Super Nintendo Entertainment System (SNES) Emulator

https://www.zsnes.com/

Interactively transferring files to a remote machine while using ssh

http://zssh.sourceforge.net/

Fast real-time compression algorithm

https://facebook.github.io/zstd/

Utilities for searching in bzip2, gzip, lzip, and xz archives

http://www.nongnu.org/zutils/zutils.html