Improved version of Icculus Quake II

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

Command-line program for downloading videos from various platforms

https://github.com/yt-dlp/yt-dlp

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/

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/

Zangband (Zelazny Angband) with color, X11 support

http://www.zangband.org/

The zed attack proxy

https://www.zaproxy.org/

Dynamically typed N-D expression system based on xtensor

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

Nibbles-like multiplayer game

https://zatacka.sourceforge.net/

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/

Barcode generator (library and utilities)

https://www.zint.org.uk/

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/

Library for the Signal Private Group System

https://github.com/signalapp/zkgroup

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

Client specific buffers module for ZNC

https://wiki.znc.in/Clientbuffer

Push notification service module for ZNC

https://github.com/jreese/znc-push

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/