Access X11 selection buffer from command line

http://BSDforge.com/projects/x11/xsel/

User preference utility for X

Set the mode for an X Input Device

Set an X Input device as the main pointer

Root window parameter setting utility for X

X Session Manager

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/

Terminal emulator for the X Window System

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

Abstract network code for X

XTrap sample clients 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

Print out X-Video extension adaptor information

X Clients under Wayland

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

Dump an image of an X window

Window information utility for X

Image displayer for X

Yet another Basic for Unix and Windows

http://www.yabasic.de/

Sega Saturn system emulator

http://yabause.sourceforge.net/

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/

Yet Another Commodore +4 Emulator

http://yape.plus4.net/

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/

DocBook XSLT stylesheets for yelp

https://www.gnome.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/

Program for downloading videos from YouTube.com

https://yt-dl.org/

The Yahoo! JavaScript and CSS Compressor

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

ZBar barcode reader

http://zbar.sourceforge.net

Encrypted, secure compressed TCP tunnel

http://www.winton.org.uk/zebedee/

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

http://www.zebra.org/

Z39.50/SR server software

http://www.indexdata.dk/zebra/

Event logging framework

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

Framework for developing PHP web applications

https://framework.zend.com/

Display GNOME dialogs from the command line

Zetaback ZFS backup and recovery management system

https://github.com/omniti-labs/zetaback/

Simple way of maintaining zfs snapshots using the periodic system

http://www.neces.com/blog/technology/integrating-freebsd-zfs-and-periodic-snapshots-and-scrubs

ZFS Snapshot Replication Script

https://github.com/leprechau/zfs-replicate

Tool to sieve ZFS snapshots as per given spec a la 'pdumpfs-clean'

https://github.com/knu/zfs-snapshot-clean

Automatic ZFS snapshot management tool

http://marcin.studio4plus.com/en/zfs-file-history/howto.html

Display human-readable ZFS statistics

http://www.vx.sk/zfs-stats/

Simple sh script to make zfs rolling snaphosts with cron

https://github.com/graudeejs/zfSnap/

OpenSolaris-compatible auto snapshotting for ZFS

https://github.com/bdrewery/zfstools

Small emacs-like text editor

http://www.gnu.org/software/zile/

Distributed computing project

http://zillion.sourceforge.net/

Create/update ZIP files compatible with PKZIP

http://infozip.sourceforge.net/Zip.html