Lightweight Open Sound Control implementation

http://liblo.sourceforge.net/

Standard lockfile library

https://github.com/miquels/liblockfile

Easy to use C/C++ seam carving library

http://liblqr.wikidot.com/

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/

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://www.gnome.org/

C library for accessing a memcache cluster

https://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/

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

https://www.libmng.com/

ModPlug mod-like music shared libraries

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

Abstraction around libmpdclient

http://gmpc.wikia.com/

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/

MusicBrainz client library for audio metadata lookup

http://musicbrainz.org/

Brush library from the MyPaint project

http://mypaint.org/

NAT-PMP lightweight library

http://miniupnp.free.fr/

C library for creating IP packets

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

HTTP/2.0 C Library

https://nghttp2.org/

Library and transmitter that implements ICE-19

https://nice.freedesktop.org

Library and transmitter that implements ICE-19

http://nice.freedesktop.org/

Library and transmitter that implements ICE-19

https://nice.freedesktop.org

C library, API for communicating with Creative and Dell audio players

http://libnjb.sourceforge.net/

Library for desktop notifications

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

Astronomical Calculation Library

http://libnova.sourceforge.net/

Library that implement Microsoft's NTLM authentication

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

C library implementing the OAuth Core standard

http://liboauth.sourceforge.net/

Replacement Objective-C runtime supporting modern Objective-C features

https://github.com/gnustep/libobjc2/

Open Fingerprint Architecture Library

http://code.google.com/p/musicip-libofa/

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

https://liboil.freedesktop.org/wiki/

Wrapping library to the System Tools Backends

Library for camera RAW files decoding

http://libopenraw.freedesktop.org/

Freedesktop synchronization framework

http://www.opensync.org/

Library for handling OpenType fonts

http://www.nongnu.org/m17n/

Portable OTR Messaging Library and toolkit

https://otr.cypherpunks.ca/

Library providing routines for paper size management

https://packages.qa.debian.org/libp/libpaper.html

Ubiquitous network traffic capture library

https://www.tcpdump.org/

libpcap wrapper library

http://netdude.sourceforge.net/