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/

Elegant music player for MPD

http://sonata.berlios.de/index.html

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/

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

Highly efficient mail filter for identifying spam

http://spamassassin.apache.org/

Traps spammers with a very slow smtp-login and return 4xx error

http://freebsdspamd.sourceforge.net/

DSP library and software FAX machine

https://www.soft-switch.org/

Extremely memory-efficient hash_map implementation

http://code.google.com/p/google-sparsehash

Spawns fastcgi applications

http://redmine.lighttpd.net/projects/spawn-fcgi

Common interface to speech synthesis

https://devel.freebsoft.org/speechd

open-source patent-free voice codec

http://www.speex.org/

Audio compression format designed for speech

http://www.speex.org/

Sphinx Full-Text Search Engine

http://sphinxsearch.com/

Gtk client and libraries for SPICE remote desktop servers

https://www.spice-space.org/

Simple Protocol for Independent Computing Environments

http://www.spice-space.org

Standalone JavaScript (1.8.5) interpreter from Mozilla

http://www.mozilla.org/js/spidermonkey/

Standalone JavaScript based from Mozilla 24-esr

http://www.mozilla.org/js/spidermonkey/

Standalone JavaScript based from Mozilla 52-esr

https://developer.mozilla.org/en-US/docs/Mozilla/Projects/SpiderMonkey

Standalone JavaScript based from Mozilla 60-esr

https://developer.mozilla.org/en-US/docs/Mozilla/Projects/SpiderMonkey

SpiderOak Client (Linux version)

https://spideroak.com/

Tools for processing SPIR-V intermediate shader language modules

https://github.com/KhronosGroup/SPIRV-Tools

Monospaced bitmap fonts for Xorg and vt(4)

https://github.com/fcambus/spleen

Tool for statically checking sourcecode for security vulnerabilities

http://www.splint.org/

Lightweight Spotify client using Qt

https://github.com/kraxarn/spotify-qt

Spotify for the terminal written in Rust

https://github.com/Rigellute/spotify-tui

Spotify daemon

https://github.com/Spotifyd/spotifyd

The Spring Framework for Java

https://www.springframework.org/

Double entry accounting system

http://www.sql-ledger.org/

Lightweight database abstraction layer

SQL database engine in a C library

http://www.sqlite.org/

SQL database engine in a C library

https://www.sqlite3.org/

Squeeze is a modern and advanced archive manager

http://squeeze.xfce.org/

HTTP Caching Proxy

http://www.squid-cache.org/

Webmail system which accesses mail over IMAP

http://www.squirrelmail.org/