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/

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

SpiderOak Client (Linux version)

https://spideroak.com/

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/

The Spring Framework for Java

https://www.springframework.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/

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/

Advanced IP/IPv6 subnet calculation and discovery

http://www.seccuris.com

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

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

Low level console graphics library

http://www.svgalib.org/

Summarizes and manages differences between local files and Subversion

http://www.incava.org/projects/svndelta/

Gnome swell-foop

https://wiki.gnome.org/Swell%20Foop

Generate wrappers for calling C/C++ code from other languages

http://www.swig.org/

Lightweight image viewer

https://github.com/muennich/sxiv

Basic Latin, Greek, Cyrillic and many Symbol blocks of Unicode

http://users.teilar.gr/~g1951d/

Mouse and keyboard sharing utility

https://symless.com/synergy

Scriptable database and system performance benchmark

https://github.com/akopytov/sysbench

Helps to install application configuration files

http://www.Courier-MTA.org/sysconftool/

Network tool designed for high performance and accurate monitoring

http://puck.nether.net/sysmon/

Backend system admin scripts for gnome-system-tools

http://www.gnome.org/projects/gst/

System Preferences application for GNUstep

http://www.gnustep.org/experience/systempreferences.html

Lossless compression library for scientific data

https://support.hdfgroup.org/doc_resource/SZIP/

Type 1 font rasterization library for Unix/X11

https://www.t1lib.org/