Library of optimized inner loops

https://liboil.freedesktop.org/wiki/

Library which provides a way to read/write CAD (DWG/DXF/DXFB) files

https://trac.osgeo.org/gdal/wiki/DWG_driver

Module music library based on OpenMPT

https://lib.openmpt.org/

Library for camera RAW files decoding

http://libopenraw.freedesktop.org/

High-level API for encoding .opus files

https://opus-codec.org/

Library for managing OS information for virtualization

https://libosinfo.org/

Off-Screen Mesa implementation of the OpenGL API

https://www.mesa3d.org/

Library for handling OpenType fonts

http://www.nongnu.org/m17n/

Portable OTR Messaging Library and toolkit

https://otr.cypherpunks.ca/

FeliCa read/write library for Sony RC-S320 USB FeliCa R/W(PaSoRi)

http://homepage3.nifty.com/slokar/pasori/libpafe.html

Ruby binding for libpafe

http://homepage3.nifty.com/slokar/pasori/libpafe-ruby.html

Eclipse Paho C Client Library for the MQTT Protocol

https://github.com/eclipse/paho.mqtt.c

IDE paneling library for GTK

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

Library providing routines for paper size management

https://packages.qa.debian.org/libp/libpaper.html

Lexer/parser utility functions for NetSurf

http://www.netsurf-browser.org/projects/libparserutils/

FeliCa read/write library for Sony RC-S320 USB FeliCa R/W(PaSoRi)

http://libpasori.sourceforge.jp/

Ubiquitous network traffic capture library

https://www.tcpdump.org/

libpcap wrapper library

http://netdude.sourceforge.net/

PCI configuration space I/O made easy

https://mj.ucw.cz/sw/pciutils/

Generic PCI access library

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

Next evolution of the Gedit plugins engine

http://live.gnome.org/libpeas

Gobject-based plugins engine (Peas 1.0 API)

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

Library for parsing, formatting, and validating phone numbers

https://github.com/google/libphonenumber

Library to deal with pinyin

https://github.com/libpinyin/libpinyin

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

GIO-styled async APIs for Flatpak

https://github.com/flatpak/libportal

GIO-styled async APIs for Flatpak

https://github.com/flatpak/libportal

GIO-styled async APIs for Flatpak

https://github.com/flatpak/libportal

GIO-styled async APIs for Flatpak

https://github.com/flatpak/libportal

GIO-styled async APIs for Flatpak (qt6 backend)

https://github.com/flatpak/libportal

Library for transforming bitmaps into vector graphics

http://potrace.sourceforge.net

Library that provides automatic proxy configuration management

https://libproxy.github.io/libproxy/

C library to handle the Public Suffix List

https://github.com/rockdaboot/libpsl

Weak aliases for pthread functions

https://xcb.freedesktop.org/

OS independent and secure pseudo-tty management

http://software.schmorp.de/pkg/libptytty.html

Backend library for the Pidgin multi-protocol messaging client

https://www.pidgin.im/

Library for generating random passwords and quality checking

https://github.com/libpwquality/libpwquality

Helper to make writing accessibility tools easier

https://api.kde.org/libqaccessibilityclient/html/index.html

Multi-purpose desktop calculator (backend library)

http://qalculate.github.io/

C library for encoding data in a QR Code symbol

http://megaui.net/fukuchi/works/qrencode/index.en.html

Library that encapsulates complex text layout logic

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

Library for manipulating raw images

https://www.libraw.org/

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

http://libwpd.sourceforge.net

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