API library for interfacing MPD

http://www.musicpd.org/libs/libmpdclient

Free library for decoding mpeg-2 and mpeg-1 video streams

http://libmpeg2.sourceforge.net/

Library for Microsoft compression formats

https://www.cabextract.org.uk/libmspack/

Multitouch Protocol Translation Library

http://bitmath.org/code/mtdev/

Media Transfer Protocol (MTP) library

http://libmtp.sourceforge.net/

MusicBrainz client library for audio metadata lookup

http://musicbrainz.org/

Brush library from the MyPaint project

http://mypaint.org/

SOFA (Spatially Oriented Format for Acoustics) file reader

https://github.com/hoene/libmysofa

NAT-PMP lightweight library

http://miniupnp.free.fr/

Neighbor discovery library

https://github.com/jpirko/libndp

C library for creating IP packets

https://sourceforge.net/projects/libnet-dev/

Client library for accessing NFS shares over a network

https://github.com/sahlberg/libnfs

HTTP/2.0 C Library

https://nghttp2.org/

HTTP/3 C Library

https://nghttp2.org/nghttp3/ https://github.com/ngtcp2/nghttp3

Implementation of QUIC protocol

https://nghttp2.org/ngtcp2/ https://github.com/ngtcp2/ngtcp2

Library and transmitter that implements ICE-19

http://nice.freedesktop.org/

Library and transmitter that implements ICE-19

http://nice.freedesktop.org/

Library for desktop notifications

http://www.galago-project.org/specs/notification/index.php

Astronomical Calculation Library

http://libnova.sourceforge.net/

NetSurf BMP decoder

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

NetSurf GIF Decoder

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

NetSurf public suffix list handling

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

NetSurf generalised utility library

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

Library that implement Microsoft's NTLM authentication

https://www.nongnu.org/libntlm/

Number to number name and money text conversion libraries

https://numbertext.org

C library implementing the OAuth Core standard

http://liboauth.sourceforge.net/

Replacement Objective-C runtime supporting modern Objective-C features

OpenSource implementation of the OFX (Open Financial eXchange)

http://libofx.sourceforge.net/

Ogg bitstream library

https://www.xiph.org/ogg/

Library of optimized inner loops

http://liboil.freedesktop.org

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

http://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

http://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

http://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