2D CAD system

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

Open Source Spotify client library

https://github.com/librespot-org/librespot

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

GNOME REST library

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

TLS library designed to ease writing foolproof applications

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

Base library for writing document import filters

https://libwpd.sourceforge.net

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 parsing and rendering SVG vector-graphic files

http://live.gnome.org/LibRsvg

Library for parsing and rendering SVG vector-graphic files

https://live.gnome.org/LibRsvg

Library for delta compression of streams

https://github.com/librsync/librsync

RT Topology Library

https://git.osgeo.org/gitea/rttopo/librttopo

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/

C++ implementation of 3mf loading/python bindings

https://github.com/Ultimaker/libSavitar

Library to access the secret service API

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

Connect and transmit data to an Icecast media streaming server

http://www.icecast.org/

GTK4 widget to display maps

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

Commodore SID-chip emulator library to play SID music files

Callback Framework for C++

http://libsigc.sourceforge.net/

Callback Framework for C++

https://libsigcplusplus.github.io/libsigcplusplus/index.html

Implementation of the Signal client protocol

https://github.com/signalapp/libsignal-client

Handling page faults in user mode

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

Complete 2D graphic library for drawing Text, Geometries, and Images

https://skia.org/

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

https://github.com/ueno/libskk

Routines for rapid alpha-numeric terminal applications development

http://www.s-lang.org/

Routines for rapid alpha-numeric terminal applications development

http://www.jedsoft.org/slang/

General purpose TCP-IP emulator library

https://gitlab.freedesktop.org/slirp/libslirp

Library to access SMI MIB information

https://www.ibr.cs.tu-bs.de/projects/libsmi/

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

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

http://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

http://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

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 SSH1 and SSH2 protocol

http://www.libssh.org/

Library implementing the SSH2 protocol

http://www.libssh2.org/

Library to build a filter for old StarOffice's documents

https://github.com/fosnola/libstaroffice

Provides a useful interface to system statistics

https://libstatgrab.org/

Wrapper providing SunOS NFSv4 ACL API