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/

Generic script compiler

http://www.datsi.fi.upm.es/~frosal/frosal.html

Font dumper for the X font server

https://www.x.org/

HTTP regression testing and benchmarking utility

http://www.joedog.org/JoeDog/Siege

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

Small (150k), fast, perl-based IRC client

http://www.iagora.com/~espel/sirc.html

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/

SLang-based newsreader

http://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

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

Session Manager Proxy

Fast compressor/decompressor library

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

Softflowd is flow-based network traffic analyser with export

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

http://spamassassin.apache.org/

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

http://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

http://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

http://www.splint.org/

The Spring Framework for Java

https://www.springframework.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/

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/