Advanced IRC bouncer

https://wiki.znc.in/ZNC

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/

Fish-like autosuggestions for Zsh

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

Interactively transferring files to a remote machine while using ssh

http://zssh.sourceforge.net/

Fast real-time compression algorithm

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

C++ header-only ZLib wrapper

https://github.com/mateidavid/zstr

A file transfer program

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

Platform-independent object-oriented C++ threading library

http://zthread.sourceforge.net/

Simple ncurses based pseudo-3D driving game

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

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

Easily and reliably transfer ZFS filesystems

https://github.com/allanjude/zxfer

ZFS Administration GUI

https://github.com/manoeldesouza

Library to provide transparent read access to zipped files

https://github.com/gdraheim/zziplib