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/

Command-line YAML and XML processor, jq wrapper for YAML/XML documents

https://github.com/kislyuk/yq

YouTube downloader written in Go

https://github.com/rylio/ytdl

Unpack data in MS Outlook TNEF format

https://github.com/Yeraze/ytnef

Yubico PIV tool

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

The Yahoo! JavaScript and CSS Compressor

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

Tool to decode yEnc encoded files

http://yydecode.sf.net/

Dynamically typed N-D expression system based on xtensor

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

Puzzle game where the player has to arrange balls in triplets

https://zaz.sourceforge.net/

ZBar barcode reader

http://zbar.sourceforge.net

Compressed file format that splits the file into independent chunks

https://github.com/zchunk/zchunk

Source port for Doom-engine games

https://www.zdoom.org/

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

http://www.zebra.org/

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

Simpler web photo gallery

https://www.zenphoto.org/

Network virtualization everywhere

https://www.zerotier.com/

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

Zoner's Half-Life compile tools

http://zhlt.info/

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

Produce .ZIP file from two other ones with the best compressed files

http://www.advsys.net/ken/utils.htm

Tool for inspecting the contents of a compressed archive

http://xanthippe.dyndns.org/Zipper/

Tool to manage keys and signatures for DNSSEC-zones

https://www.hznet.de/dns/zkt/

Fork of the zlib data compression library

https://github.com/zlib-ng/zlib-ng

Internet scanner

https://zmap.io/

ZnapZend is a ZFS centric backup tool

https://www.znapzend.org/

Advanced IRC bouncer

https://wiki.znc.in/ZNC

Fast static site generator

https://www.getzola.org/

DNS notify sender written in C

http://www.morettoni.net/zonenotify.en.html

Manipulate archives of files in compressed form

Coordination Service for Distributed Applications

https://zookeeper.apache.org/

Zoom videoconferencing client (CAVEAT: Sound doesn't yet work)

https://zoom.us/

Zopfli Compression Algorithm

https://github.com/google/zopfli

Swiss army knife for the serious backup manager

https://github.com/fcorbelli/zpaqfranz

Zooming photo album generator

http://namazu.org/~satoru/zphoto/

ZFS based replication and failover solution

http://www.bolthole.com/solaris/zrep/

Z shell

https://www.zsh.org/

Tool for managing Zshell plugins inspired by Vundle

https://github.com/zsh-users/antigen

Fish-like autosuggestions for Zsh

https://github.com/zsh-users/zsh-autosuggestions