GObject based library for rendering XPS documents

https://wiki.gnome.org/action/show/Projects/libgxps

Library with GTK+ widgets for mobile phones

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

ISO/IEC 23008-12:2017 HEIF file format de- and encoder

https://github.com/strukturag/libheif

HTML5 parser library

https://www.netsurf-browser.org/projects/hubbub/

Implementation of the IETF Calendaring and Scheduling protocols

https://github.com/libical

Character set conversion library

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

ID3 tags library (part of MAD project)

http://mad.sourceforge.net/

Internationalized Domain Names command line tool

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

Implementation of IDNA2008 internationalized domain names

https://www.gnu.org/software/libidn/#libidn2 https://gitlab.com/libidn/libidn2

C library that supports plugin printer driver for Ghostscript

http://www.linuxprinting.org/ijs/

Image Quantization Library

https://pngquant.org/lib/

Library to support generic input method implementation

https://github.com/fcitx/libime

Library to support Jyutping through libime

https://github.com/fcitx/libime-jyutping

Library to communicate with Apple iOS devices

http://www.libimobiledevice.org

Common code used by the libimobiledevice project

https://www.libimobiledevice.org

Kevent based inotify compatible library

https://github.com/libinotify-kqueue/libinotify-kqueue

Generic input library

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

ODBC 3.x driver manager, for universal data source access

http://www.iodbc.org/

Library for manipulating IPTC metadata stored in image files

http://libiptcdata.sourceforge.net/

Library for Evation's Irman infrared receiver

https://libirman.sourceforge.io/

Libburnia ISO9660 filesystem creation library

https://dev.lovelyhq.com/libburnia/web/wikis/home

Standalone client/server 9P library

https://libs.suckless.org/deprecated/libixp/

SIMD-accelerated JPEG codec library, provides libTurboJPEG

https://libjpeg-turbo.org/

JPEG XL reference encoder/decoder

https://jpeg.org/jpegxl/

KDE CDDB library

https://www.kde.org/

KDE library for interfacing with audio CDs

https://www.kde.org/

LibRaw interface for KDE

https://www.digikam.org/

Libraries used by KDE games

https://www.kde.org/applications/games/

Library for reading and writing vocabulary files

https://edu.kde.org/

Exiv2 library interface for KDE

https://www.digikam.org/

Library for the Mahjongg Solitaire for KDE

https://www.kde.org/applications/games/kmahjongg/

Engine to interpret compiled KMFL keyboard tables

http://kmfl.sourceforge.net/

Reference implementation of OGC KML 2.2

https://github.com/libkml/libkml

Kolab XML Format Schema Definitions Library

https://kolab.org/

Library to make X.509 certificates

https://www.gnupg.org/aegypten2/

Interface library to access tags for identifying languages

http://tagoh.bitbucket.org/liblangtag/

Lightweight KMS plane library

https://gitlab.freedesktop.org/emersion/libliftoff

Lightweight Open Sound Control implementation

http://liblo.sourceforge.net/

Standard lockfile library

https://github.com/miquels/liblockfile

Easy to use system logging library

https://www.liblogging.org/

Easy to use C/C++ seam carving library

http://liblqr.wikidot.com/

Library for manipulating RDF files describing LADSPA plugins

https://github.com/swh/LRDF

System independent dlopen wrapper

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

LZ4 compression library, lossless and very fast

http://www.lz4.org/

Provides a few data structures and helpful functions

https://github.com/cheusov/libmaa

Libmad library (part of MAD project)

https://www.underbit.com/products/mad/

MATE keyboard shared library

https://mate-desktop.org/

Mixer library for MATE desktop

https://github.com/mate-desktop/libmatemixer

Library to accessing online weather informations

https://mate-desktop.org/

Extensible Multimedia Container Format

https://www.matroska.org/