Library to build higher-level cryptographic tools

https://github.com/jedisct1/libsodium

SOAP (Simple Object Access Protocol) implementation in C

https://libsoup.org/

HTTP client/server library for GNOME

High quality, one-dimensional sample-rate conversion library

http://sourceforge.net/p/soxr/

Small library for rendering Postscript documents

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

Utility library for creating Spiel speech providers

https://project-spiel.org/libspeechprovider/

Spellcheck library for GTK 4

https://gitlab.gnome.org/GNOME/libspelling

Sender Rewriting Scheme 2 C Implementation

https://www.libspf2.org/

Implements the server side of the SPICE protocol

http://spice-space.org/

Speech synthesis client library

https://project-spiel.org/libspiel/

Library to convert clothoid splines into Bezier splines

https://github.com/fontforge/libspiro

Simple PNG decoding and encoding library

https://libspng.org/

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/

Provides a useful interface to system statistics

https://libstatgrab.org/

Wrapper providing SunOS NFSv4 ACL API

Parser for SVG content in files or buffers

http://cairographics.org/libsvg

Sysctl MIB-Tree API (improved version)

https://gitlab.com/alfix/sysctlmibinfo2/

GNU libc's sysinfo port for FreeBSD

https://github.com/bsdimp/libsysinfo

Unified library for Chinese text processing

https://libtabe.sourceforge.net/

ASN.1 structure parser library

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

Library for communicating with Apple's Tatsu Signing Server

https://www.libimobiledevice.org

Language guessing by N-Gram-Based Text Categorization

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

Text styling library

https://www.gnu.org/software/gettext/libtextstyle/manual/

Thai language support library

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

Theora video codec for the Ogg multimedia streaming system

https://www.theora.org/

Low-level C Library to implement TLSRPT into a MTA

https://github.com/sys4/libtlsrpt

Comprehensive, modular, and portable cryptographic toolkit

https://libtom.net/

Comprehensive, modular, and portable mathematical routines

https://libtom.net/

TOMOE GTK library

http://tomoe.sourceforge.jp/

Generic shared library support script

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

BitTorrent Library

https://github.com/rakshasa/libtorrent

C++ library implementing a BitTorrent client

https://libtorrent.org https://github.com/arvidn/libtorrent

C++ library implementing a BitTorrent client

https://libtorrent.org https://github.com/arvidn/libtorrent

Lightweight fully POSIX compliant regexp matching library

https://laurikari.net/tre/

User space caching library

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

Universal configuration library parser

https://github.com/vstakhov/libucl

libudev-compatible interface for devd

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

UDF reader for media players

https://code.videolan.org/videolan/libudfread/

Library to manipulate storage devices, from udisks

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

Implementation of the line and word breaking algorithm for Unicode

https://vimgadgets.sourceforge.net/libunibreak/

Library for manipulating Unicode characters and strings

http://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

Library for single instance applications

http://live.gnome.org/LibUnique

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