WebRTC Data Channels, WebRTC Media Transport, and WebSockets library

https://github.com/paullouisageneau/libdatachannel

Double-array trie implementation library

https://linux.thai.net/projects/datrie/

Companion library to GObject and GTK

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

GLib and Gtk Implementation of the DBusMenu protocol

https://launchpad.net/libdbusmenu

Qt5 implementation of the DBusMenu protocol

https://launchpad.net/libdbusmenu-qt

1394-based DC Control Library

https://damien.douxchamps.net/ieee1394/libdc1394/

Free DTS Coherent Acoustics decoder

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

Open source h.265 video codec

https://www.libde265.org

Client-side decorations library for Wayland client

https://gitlab.freedesktop.org/libdecor/libdecor

Fast, whole-buffer DEFLATE-based compression library

https://github.com/ebiggers/libdeflate

Generic Device Query and Monitor interface

https://github.com/freebsd/libdevq

Library supporting Deferred Execution for GTK applications

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

Library to generate arbitrary sequence diffs

https://github.com/kristapsdz/libdiff

Library for creating MusicBrainz DiscIDs

http://musicbrainz.org/doc/libdiscid

EDID and DisplayID library

https://gitlab.freedesktop.org/emersion/libdisplay-info

Library for reading and writing Data Matrix barcodes

http://libdmtx.sourceforge.net/

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/

Document Object Model library

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

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/

Library for MPEG TS and DVB PSI tables decoding and generation

http://www.videolan.org/developers/libdvbpsi.html

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

EBML (Extensible Binary Meta Language), sort of binary version of XML

https://www.matroska.org/

Collection of C++/Qt classes designed to use the APIs of The Echo Nest

https://github.com/lfranchi/libechonest

Command line editor library

http://thrysoee.dk/editline/

Library for Emulated Input

https://gitlab.freedesktop.org/libinput/libei

Public ELF file access library similar to libelf(3) in Solaris

https://web.archive.org/web/20181111033959/www.mr511.de/software/english.html

Enhanced Metafile Library

http://libemf.sourceforge.net/

Small epoll implementation using kqueue

https://github.com/jiixyj/epoll-shim

Library to handle OpenGL function pointer management

https://github.com/anholt/libepoxy

Library for generating documents in ePub format

http://libepubgen.sourceforge.net/

Liberation fonts from Red Hat to replace MS TTF fonts

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

C++11 simple, header-only, TIFF reader

https://github.com/libertiff/libertiff

Library for posting electronic mail

http://www.stafford.uklinux.net/libesmtp/

Library for some string essentials

https://libestr.adiscon.com/

Mail framework for C language

http://www.etpan.org/libetpan.html

Full-featured and high-performance event loop library

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

Linux Event Device library

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

API for executing callback functions on events or timeouts

https://libevent.org/

Library to read digital camera file meta-data

https://libexif.github.io/

Application library for the Xfce desktop environment

https://www.xfce.org/

Language guessing by N-Gram-Based Text Categorization

https://wiki.documentfoundation.org/Libexttextcat

X virtual keyboard library

https://www.yoctoproject.org/tools-resources/projects/matchbox

Video encoding library

http://fame.sourceforge.net/

Fork of json-c (JSON implementation in C)

https://github.com/rsyslog/libfastjson

Foreign Function Interface

https://sourceware.org/libffi/