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)

http://mad.sourceforge.net/

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/

Library for the MaxMind DB file format used for GeoIP2

https://github.com/maxmind/libmaxminddb

Multi-cipher cryptographic library (used in PHP)

http://mcrypt.sourceforge.net/

Library for handling media art

https://github.com/GNOME/libmediaart

MediaInfo library

https://mediaarea.net/en/MediaInfo https://github.com/MediaArea/MediaInfoLib

The C library for accessing a memcache cluster

http://people.FreeBSD.org/~seanc/libmemcache/

C and C++ client library to the memcached server

http://libmemcached.org

Set of utilities to benchmark productivity of system calls

http://hub.opensolaris.org/bin/view/Project+libmicro/

C library for embedding HTTP server functionality

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

MikMod Sound Library

http://mikmod.sourceforge.net/

Library providing Sendmail Mail Filter API

http://www.sendmail.com/sm/open_source/milter/

Multiple-image Network Graphics (MNG) reference library

http://www.libmng.com/

Modbus library

https://libmodbus.org/

Library and tools to access the Mac OS disk image formats

https://github.com/libyal/libmodi

ModPlug mod-like music shared libraries

http://modplug-xmms.sourceforge.net/

Perl module to manipulate filenames containing music metadata

https://erislabs.net/ianb/projects/libmp3-archive-perl/

Abstraction around libmpdclient

http://cms.qballcow.nl/index.php?page=libmpd

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

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

Library and tools for parsing Microsoft Publisher file format

https://wiki.documentfoundation.org/DLP/Libraries/libmspub

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/

Import library for some old mac text documents

https://libmwaw.sourceforge.net/

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

http://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

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

NetSurf GIF Decoder

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

NetSurf public suffix list handling

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

NetSurf generalised utility library

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

Library that implement Microsoft's NTLM authentication

http://www.nongnu.org/libntlm/

Number to number name and money text conversion libraries

https://numbertext.org