X Toolkit library

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

Network API translation layer for X applications and libraries

http://www.freedesktop.org/Software/xlibs

X Test extension

http://www.freedesktop.org/Software/xorg

X Video Extension library

http://www.freedesktop.org/Software/xlibs

X Video Extension Motion Compensation library

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

X DGA Extension

http://www.freedesktop.org/Software/xorg

X XF86-Misc Extension

http://www.freedesktop.org/Software/xorg

X Vidmode Extension

http://www.freedesktop.org/Software/xorg

Free library for decoding ATSC A/52 streams, aka AC-3

http://liba52.sourceforge.net/

3GPP Adaptive Multi-Rate - Wideband (AMR-WB) Speech Codec

http://www.penguin.cz/~utx/amr

Portable audio output library

https://www.xiph.org/ao/

Library for high-performance 2D graphics

http://www.levien.com/libart/

IPC library used by GnuPG and gpgme

https://gnupg.org/related_software/libassuan/index.en.html

Sound library for SGI audio file

http://www.68k.org/~michael/audiofile/

Standard C resolver library

https://www.isc.org/software/libbind

Component and compound document system for GNOME2

http://www.gnome.org/

Libburnia library to read/write optical discs

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

Graphics library that outputs text instead of pixels

http://caca.zoy.org/

Library for playing audio CDs

http://libcdaudio.sourceforge.net/

Library to access data on a CDDB server

http://libcddb.sourceforge.net/

Compact Disc Input and Control Library

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

Unit test framework for C

http://check.sourceforge.net/

Configuration file parsing library

http://savannah.nongnu.org/projects/confuse/

CSS2 parsing library

http://www.freespiders.org/projects/libcroco/

Lightweight C library that eases the writing of UNIX daemons

http://0pointer.de/lennart/projects/libdaemon/

Free DTS Coherent Acoustics decoder

https://www.videolan.org/developers/libdca.html

Library for creating MusicBrainz DiscIDs

http://musicbrainz.org/doc/libdiscid

DMX extension library

http://www.freedesktop.org/Software/xorg

Simple interface to low level networking routines

https://github.com/5u623l20/libdnet/

Standard library for Window Maker dock apps

http://solfertje.student.utwente.nl/~dalroi/libdockapp/

Userspace interface to kernel Direct Rendering Module services

https://dri.freedesktop.org/

Alsa compatibility library to build DSSI

http://www.smbolton.com/linux.html

Quasar DV codec (libdv): software codec for DV video encoding format

http://libdv.sourceforge.net/

Portable abstraction library for DVD decryption

https://www.videolan.org/developers/libdvdcss.html

Videolan version of the libdvdnav project

https://www.videolan.org/developers/libdvdnav.html

MPlayer version of the libdvdread project

https://www.videolan.org/developers/libdvdnav.html

Enhanced Metafile Library

http://libemf.sourceforge.net/

Liberation fonts from Red Hat to replace MS TTF fonts

https://github.com/liberationfonts/liberation-fonts

Full-featured and high-performance event loop library

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

API for executing callback functions on events or timeouts

https://libevent.org/

API for executing callback functions on events or timeouts

http://libevent.org/

Library for inspecting program's backtrace

http://www.gnu.org/software/libc/manual/html_node/Backtraces.html

Library to read digital camera file meta-data

https://libexif.github.io/

Video encoding library

http://fame.sourceforge.net/

Foreign Function Interface

https://sourceware.org/libffi/

The fontenc Library

https://www.x.org/

Library routines for working with Flashpix images

https://www.i3a.org/

General purpose crypto library based on code used in GnuPG

https://www.gnupg.org/software/libgcrypt/index.html

GDI+ API for System.Windows.Forms in Mono

https://www.mono-project.com/docs/gui/libgdiplus/

A flexible drawing library

http://www.ggi-project.org/