Library for parsing, formatting, and validating phone numbers

https://github.com/google/libphonenumber

Reusable library for GPU-accelerated video/image rendering

https://code.videolan.org/videolan/libplacebo

Library to handle Apple Property List format

http://www.libimobiledevice.org

Library that provides automatic proxy configuration management

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

C library to handle the Public Suffix List

https://github.com/rockdaboot/libpsl

This library provides weak aliases for pthread functions

http://xcb.freedesktop.org/

Library for generating random passwords and quality checking

https://fedorahosted.org/libpwquality/

Multi-purpose desktop calculator (backend library)

http://qalculate.sourceforge.net

Library for parsing QuarkXPress documents

https://wiki.documentfoundation.org/DLP/Libraries/libqxp

Library that encapsulates complex text layout logic

https://github.com/HOST-Oman/libraqm

2D CAD system

https://librecad.org/ https://github.com/LibreCAD/LibreCAD

Free version of the SSL/TLS protocol forked from OpenSSL

http://www.libressl.org/

Easy access to RESTful web services

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

TLS library designed to ease writing foolproof applications

https://git.causal.agency/libretls/about/

Standalone port/fork of Mednafen PSX to the Libretro API

https://github.com/libretro/beetle-psx-libretro

Unofficial build of FCEU Ultra by CaH4e3

https://github.com/libretro/libretro-fceumm

Enhanced port of Genesis Plus

https://github.com/libretro/Genesis-Plus-GX

Fast MegaDrive/MegaCD/32X emulator

https://github.com/libretro/picodrive

Rime Input Method Engine, the core library

https://rime.github.io/

Library for Reliable Internet Stream Transport (RIST) protocol

https://code.videolan.org/rist/librist

Library for delta compression of streams

https://github.com/librsync/librsync

C/C++ implementation of a Sass compiler

http://libsass.org/

Callback Framework for C++

http://libsigc.sourceforge.net/

Library to deal with Japanese Kana-to-Kanji conversion method

https://github.com/ueno/libskk

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

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

Library to build higher-level cryptographic tools

https://github.com/jedisct1/libsodium

Sender Rewriting Scheme 2 C Implementation

http://www.libspf2.org/

Library to convert clothoid splines into Bezier splines

http://libspiro.sourceforge.net/

Library for communicating with spacenavd or 3dxsrv

https://spacenav.sourceforge.net/

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 to build a filter for old StarOffice's documents

https://github.com/fosnola/libstaroffice

Sysctl MIB-Tree API (improved version)

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

Library for communicating with Apple's Tatsu Signing Server

https://www.libimobiledevice.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/

BitTorrent Library written in C++

https://github.com/rakshasa/libtorrent

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/

Reference codec for the Ultra HDR format

https://developer.android.com/media/platform/hdr-image-format https://github.com/google/libultrahdr

Implementation of the line and word breaking algorithm for Unicode

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

Library of Unicode annotation data

http://libuninameslist.sourceforge.net/

Generic stack unwinding library

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

Communication interface library for usbmuxd

http://marcansoft.com/blog/iphonelinux/usbmuxd

The uTorrent Transport Protocol library and sample utilities

https://github.com/bittorrent/libutp

Library to generate or parse 128-bit Universally Unique IDs (UUIDs)

https://github.com/util-linux/util-linux