GNOME application for managing encryption keys (PGP, SSH)

https://wiki.gnome.org/Apps/Seahorse

Minimal seat management daemon and universal library

https://git.sr.ht/~kennylevinsen/seatd

Secure data deletion toolkit

https://www.thc.org/

Filesystem in userspace with transparent encryption and decryption

https://github.com/netheril96/securefs

Checks installed packages for vulnerabilities

http://github.com/laffer1/security-advisory-client/

Scaling, colorspace conversion, and dithering library

https://github.com/sekrit-twc/zimg

Extensive german HTML reference

http://www.selfhtml.org/

Reliable, highly configurable mail transfer agent with utilities

https://www.sendmail.org/

Perl-script to send xmpp (jabber) messages

https://sendxmpp.hostname.sk/

C library for RDF syntax

https://drobilla.net/software/serd

Serf HTTP client library

https://serf.apache.org/

Manage utmp/wtmp entries for non-init X clients

Set the keyboard using the X Keyboard Extension

https://www.freedesktop.org/Software/xorg

Wrapper library around CGAL for PostGIS

https://oslandia.gitlab.io/SFCGAL/

Set of utilities that send SCSI commands to devices

http://sg.danny.cz/sg/sg3_utils.html

Fonts from the SGI ProPack 1.4 (originally for Linux)

http://oss.sgi.com/projects/sgi_propack/

Generates groff and HTML from linuxdoc and docbook SGML documents

SGML parser

GLSL/HLSL to SPIR-V shader compiler

https://github.com/google/shaderc

Interoperability between desktops and collaborative environments

http://oscaf.sourceforge.net

MIME types database from the freedesktop.org project

https://www.freedesktop.org/wiki/Software/shared-mime-info/

Collection of shareware ATM fonts from the CICA archives

Pack, send, and unpack shell archives; synchronize via e-mail

http://www.gnu.org/s/sharutils/

Generic script compiler

http://www.datsi.fi.upm.es/~frosal/frosal.html

Benchmark utility for POSIX shell comparison

https://github.com/shellspec/shellbench

Font dumper for the X font server

https://www.x.org/

xUnit based unit testing for Unix shell scripts

https://github.com/kward/shunit2/

Text based Hotline Communications client for Unix

http://synhxd.sourceforge.net/

HTTP regression testing and benchmarking utility

http://www.joedog.org/JoeDog/Siege

MacOS High Sierra like theme for GTK 3, GTK 2 and Gnome-Shell etc

https://github.com/vinceliuice/Sierra-gtk-theme

GPG/PGP Keyring Graph Generator

http://www.chaosreigns.com/code/sig2dot/

Sigar system information API

https://support.hyperic.com/display/SIGAR/Home

Command-line and D-Bus interface for Signal and libsignal-service-java

https://github.com/AsamK/signal-cli

FTS5 extension for signal_tokenizer

https://github.com/signalapp/Signal-FTS5-Extension

Create and verify cryptographic signatures

https://github.com/leahneukirchen/outils/

Sigrok firmware

https://www.sigrok.org/wiki/Firmware

Cypress FX2 firmware for hardware logic analyzers

https://www.sigrok.org/wiki/Fx2lafw

Sigrok firmware extraction utilities

https://www.sigrok.org/wiki/Firmware

Secure Internet Live Conferencing (SILC) network toolkit

http://www.silcnet.org/

Font engine for complex non-Roman writing systems

http://silgraphite.sourceforge.net/

Simple MTP fuse filesystem driver

https://github.com/phatina/simple-mtpfs

simple calendar and agenda application

http://wiki.gnustep.org/index.php/SimpleAgenda.app

C++ library providing a simple API to INI-style configuration files

https://github.com/brofield/simpleini

Extremely simple IRC client

http://www.suckless.org/programs/sic.html

Small (150k), fast, perl-based IRC client

http://www.iagora.com/~espel/sirc.html

Maintains remote websites, uses FTP or WebDAV to sync up w/ local copy

https://www.manyfish.co.uk/sitecopy/

General purpose C libraries used by skarnet.org software

https://www.skarnet.org/software/skalibs/

Slack client for your terminal

https://github.com/erroneousboat/slack-term

SIMD Library for Evaluating Elementary Functions, vectorized libm

https://sleef.org

Tool for checking, and diagnosing DNS zones