Secret Rabbit Code: a Sample Rate Converter for audio

http://www.mega-nerd.com/SRC/

C/C++ implementation of a Sass compiler

http://libsass.org/

Library to access the secret service API

https://wiki.gnome.org/Projects/Libsecret/

Extension widgets for GTK+

http://releases.chipx86.com/libsexy/libsexy/

Connect and transmit data to an Icecast media streaming server

https://icecast.org/

Commodore SID-chip emulator library to play SID music files

Callback Framework for C++

http://libsigc.sourceforge.net/

Callback Framework for C++

http://libsigc.sourceforge.net/

Handling page faults in user mode

http://www.gnu.org/software/libsigsegv/

Routines for rapid alpha-numeric terminal applications development

https://www.jedsoft.org/slang/

Reading and writing files containing sampled sound (like WAV or AIFF)

http://www.mega-nerd.com/libsndfile/

Personal social data server.

Library to build higher-level cryptographic tools

https://github.com/jedisct1/libsodium

SOAP (Simple Object Access Protocol) implementation in C

https://libsoup.org/

SOAP (Simple Object Access Protocol) implementation in C

Small library for rendering Postscript documents

https://www.freedesktop.org/wiki/Software/libspectre/

Sender Rewriting Scheme 2 C Implementation

https://www.libspf2.org/

Implements the server side of the SPICE protocol

https://spice-space.org/

Library to convert clothoid splines into Bezier splines

https://github.com/fontforge/libspiro

Secure RTP (SRTP) Reference Implementation

https://github.com/cisco/libsrtp

Library implementing the SSH2 protocol

https://www.libssh.org/

Library implementing the SSH2 protocol

https://www.libssh2.org/

Strigi desktop search: streams library

http://www.vandenoever.info/software/strigi/

Wrapper providing SunOS NFSv4 ACL API

Parser for SVG content in files or buffers

http://cairographics.org/libsvg

ASN.1 structure parser library

https://www.gnu.org/software/libtasn1/

Flexible Communications Framework

http://telepathy.freedesktop.org/

Language guessing by N-Gram-Based Text Categorization

https://software.wise-guys.nl/libtextcat/

Thai language support library

https://linux.thai.net/projects/libthai/

Theora video codec for the Ogg multimedia streaming system

https://www.theora.org/

Generic shared library support script

https://www.gnu.org/software/libtool/

BitTorrent Library

https://github.com/rakshasa/libtorrent

Lightweight fully POSIX compliant regexp matching library

https://laurikari.net/tre/

Integer-only fully Ogg Vorbis compliant decoder library

https://xiph.org/vorbis/

User space caching library

http://mercurial.creo.hu/repos/libublio

libudev-compatible interface for devd

https://github.com/wulf7/libudev-devd

Library for manipulating Unicode characters and strings

https://www.gnome.org/

Library of Unicode annotation data

http://libuninameslist.sourceforge.net/

Library for converting Unicode strings to numbers

http://billposer.org/Software/libuninum.html

Unicode string library

https://www.gnu.org/software/libunistring/

Library to work with RAR archivies

http://www.rarsoft.com/

Generic stack unwinding library

https://www.nongnu.org/libunwind/

Userspace read-copy-update (RCU) data synchronization library

http://lttng.org/urcu

Communication interface library for usbmuxd

http://www.libimobiledevice.org

The uTorrent Transport Protocol library and sample utilities

https://github.com/bittorrent/libutp

Multi-platform support library with a focus on asynchronous I/O

https://github.com/libuv/libuv

Video4Linux library

https://linuxtv.org/wiki/index.php/V4l-utils

VAAPI wrapper and dummy driver

https://github.com/intel/libva

VDPAU wrapper and tracing library

http://freedesktop.org/wiki/Software/VDPAU

Abstraction library that sits between apps and visual plugin

http://libvisual.sourceforge.net/