SixXS IPv6 TIC+ tunnel broker heartbeat client

http://www.sixxs.net/tools/aiccu/

Fully automated, active web application security reconnaissance tool

http://code.google.com/p/skipfish

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/

SLang-based newsreader

https://slrn.sourceforge.net/

SIP scanner and identifier

http://www.wormulon.net/

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

https://www.smartmontools.org

Free MPEG1 video player library with sound support

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

Session Manager Proxy

Fast compressor/decompressor library

https://google.github.io/snappy/

Flow-based network traffic analyzer and exporter

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

QT4 RDF framework

http://soprano.sourceforge.net

Open-source audio processing library

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

Highly efficient mail filter for identifying spam

https://spamassassin.apache.org/

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

https://freebsdspamd.sourceforge.net/

Spawns fastcgi applications

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

SPDY protocol version 2, 3 and 3.1 implementation in C

http://tatsuhiro-t.github.io/spdylay/

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

https://sphinxsearch.com/

Standalone JavaScript based from Mozilla 17-esr

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

Standalone JavaScript based from Mozilla 24-esr

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

SpiderOak Client (Linux version)

https://spideroak.com/

Tool for statically checking sourcecode for security vulnerabilities

https://www.splint.org/

The Spring Framework for Java

https://www.springframework.org/

SQL database engine in a C library (TCL flavor)

https://www.sqlite.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

https://www.squirrelmail.org/

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

https://www.sshguard.net/

Protect hosts from brute force attacks against ssh and other services using ipfilter

http://sshguard.sourceforge.net

Protect hosts from brute force attacks against ssh using ipfw

http://sshguard.sourceforge.net

Protect hosts from brute force attacks against ssh using pf

http://sshguard.sourceforge.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/

Advanced IP/IPv6 subnet calculation and discovery

http://www.seccuris.com

Version control system

http://subversion.apache.org/

Allow others to run commands as root

https://www.sudo.ws/

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

http://supertuxproject.org/

Low level console graphics library

http://www.svgalib.org/

only pure Java Subversion client library in the world

http://svnkit.com/

Simplified Wrapper and Interface Generator

http://www.swig.org/

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

http://www.swig.org/

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

http://www.swig.org/

Mouse and keyboard sharing utility

https://symless.com/synergy

Helps to install application configuration files

https://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