Search using regexes for file names and text

http://searchmonkey.sourceforge.net/

Checks installed packages for vulnerabilities

http://github.com/laffer1/security-advisory-client/

Extensive german HTML reference

http://www.selfhtml.org/

Perl-script to send xmpp (jabber) messages

https://sendxmpp.hostname.sk/

Serf HTTP client library

https://serf.apache.org/

Manage utmp/wtmp entries for non-init X clients

Set the keyboard using the X Keyboard Extension

https://www.freedesktop.org/Software/xorg

Set of utilities that send SCSI commands to devices

http://sg.danny.cz/sg/sg3_utils.html

Generates groff and HTML from linuxdoc and docbook SGML documents

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

https://github.com/neurobin/shc/

Font dumper for the X font server

https://www.x.org/

HTTP regression testing and benchmarking utility

https://www.joedog.org/JoeDog/Sieg

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

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/

Theme pack for SLiM login app

https://contents.meetbsd.ir/apps/opensource/slim/

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

Samba-LDAP management and support tools

https://gna.org/projects/smbldap-tools/

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/

Super Nintendo Entertainment System(SNES) Emulator

http://www.snes9x.com/

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/

Custom rulesets for SpamAssassin

http://wiki.apache.org/spamassassin/CustomRulesets

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/

Extremely memory-efficient hash_map implementation

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

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/

Simple Protocol for Independent Computing Environments

https://www.spice-space.org

Standalone JavaScript based from Mozilla 17-esr

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

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/

Tool for statically checking sourcecode for security vulnerabilities

https://www.splint.org/

The Spring Framework for Java

https://www.springframework.org/

Lightweight database abstraction layer