Image displayer for X

Yet another Basic for Unix and Windows

http://www.yabasic.de/

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/

Z39.50/SR client and API library

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

DocBook XSLT stylesheets for yelp

https://www.gnome.org/

Program for downloading videos from YouTube.com

https://yt-dl.org/

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

http://www.zebra.org/

Event logging framework

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

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

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/

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

Tool for inspecting the contents of a compressed archive

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

A Massively Spiffy Yet Delicately Unobtrusive Compression Library

http://zlib.net/

Advanced IRC bouncer

https://wiki.znc.in/ZNC

Manipulate archives of files in compressed form

ZFS based replication and failover solution

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

Z shell

https://www.zsh.org/

Interactively transferring files to a remote machine while using ssh

http://zssh.sourceforge.net/

A file transfer program

http://zsync.moria.org.uk/

Platform-independent object-oriented C++ threading library

http://zthread.sourceforge.net/

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

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

Library to provide transparent read access to zipped files

https://github.com/gdraheim/zziplib