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/

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

Create and verify cryptographic signatures

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

Secure Internet Live Conferencing (SILC) network toolkit

http://www.silcnet.org/

Font engine for complex non-Roman writing systems

http://silgraphite.sourceforge.net/

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

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

Tool for checking, and diagnosing DNS zones

Simple facade or abstraction for various logging frameworks

https://www.slf4j.org/

Graphical login manager for X11, derived from Login.app

https://sourceforge.net/projects/slim.berlios/

Theme pack for SLiM login app

http://slim.berlios.de/themes01.php

Superior Lisp Interaction Mode for Emacs

https://common-lisp.net/project/slime/

Low-bandwidth, yet greedy and poisonous HTTP client

http://ha.ckers.org/slowloris/

SLang-based newsreader

http://slrn.sourceforge.net/

S.M.A.R.T. disk monitoring tools

https://www.smartmontools.org

Samba-LDAP management and support tools

https://gna.org/projects/smbldap-tools/

Free MPEG1 video player library with sound support

http://www.lokigames.com/development/smpeg.php3

Session Manager Proxy

Fast compressor/decompressor library

http://google.github.io/snappy/

Fast compressor/decompressor library

https://github.com/xerial/snappy-java

Small audio and MIDI framework from the OpenBSD project

http://www.sndio.org/

Lightweight network intrusion detection system

http://www.snort.org/

Snowball stemmer library for C

https://snowballstem.org/

Softflowd is flow-based network traffic analyser with export

http://www.mindrot.org/softflowd.html

SonarQube Plugin for analysis projects with an Apache Ant

http://docs.sonarqube.org/display/SONAR/Installing+and+Configuring+Ant+SonarQube+Task

Scanner component of SonarQube

https://www.sonarqube.org/

Platform for continuous inspection of code quality

http://www.sonarqube.org/

Lightweight C library for storing RDF data in memory

http://drobilla.net/software/sord/

Clean, mean and lean CD ripper for GNOME

http://www.burtonini.com/blog/computers/sound-juicer/

Open-source audio processing library

http://www.surina.net/soundtouch/

C/C++ and Java sources to HTML converter

https://www.gnu.org/software/src-highlite/source-highlight.html

Set of fonts by Adobe designed for coders

https://github.com/adobe-fonts/source-code-pro

Set of fonts by Adobe designed for user interfaces

https://github.com/adobe-fonts/source-sans-pro

Set of fonts by Adobe designed for UI

https://github.com/adobe-fonts/source-serif-pro

Custom rulesets for SpamAssassin

http://wiki.apache.org/spamassassin/CustomRulesets