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 based from Mozilla 24-esr

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

Standalone JavaScript based from Mozilla 68-esr

https://spidermonkey.dev/

Standalone JavaScript based from Mozilla 78-esr

https://spidermonkey.dev/

Standalone JavaScript based from Mozilla 91-esr

https://spidermonkey.dev/

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 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/

HTTP Caching Proxy

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

Webmail system which accesses mail over IMAP

http://www.squirrelmail.org/

Library for serialising LV2 atoms to/from RDF

http://drobilla.net/software/sratom

Secure Reliable Transport (SRT) Protocol

https://www.srtalliance.org/

Chat over SSH

https://github.com/shazow/ssh-chat

Protect hosts from brute-force attacks against SSH and other services

https://www.sshguard.net/

State Threads Library for Internet Applications

http://state-threads.sourceforge.net/

Library that supports startup notification spec from freedesktop.org

https://www.freedesktop.org/wiki/Software/startup-notification/

Monospace font created for the French magazine Etapes

https://raphaelbastide.com/steps-mono

OpenType Unicode fonts for Scientific, Technical, and Math texts

http://www.stixfonts.org/

Synthesis ToolKit in C++

https://ccrma.stanford.edu/software/stk

Command-line utility that pipes video streams

https://streamlink.github.io/

Stuffit Archive Creator and Expander

http://my.smithmicro.com/unix/stuffit/

Advanced IP/IPv6 subnet calculation and discovery

http://www.seccuris.com

Subsonic streaming media server, standalone version

http://www.subsonic.org

Version control system

http://subversion.apache.org/

Version control system, long-time-support version

http://subversion.apache.org/

Allow others to run commands as root

https://www.sudo.ws/

Font for programmers and command line users

https://www.kutilek.de/sudo-font

C library for loading and wrapping LV2 plugin UIs

http://drobilla.net/software/suil/

SuiteSparse is a set of packages for sparse matrices calculation

http://faculty.cse.tamu.edu/davis/suitesparse.html

Super Tux is a side-scroller similar to Super Mario Brothers

http://supertuxproject.org/

Tool similar to Gloobus Preview that provides quick file previews

http://www.ubuntugeek.com/gnome-sushi-quick-previewer-for-nautilus.html

X11 font designed to be small yet easily read

http://www.goof.com/pcg/marc/suxus.html