Implementation of the IETF's Calendaring and Scheduling protocols

https://github.com/libical

Character set conversion library

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

ID3 tags library (part of MAD project)

http://mad.sourceforge.net/

Internationalized Domain Names command line tool

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

Implementation of IDNA2008 internationalized domain names

https://www.gnu.org/software/libidn/#libidn2

C library that supports plugin printer driver for Ghostscript

http://www.linuxprinting.org/ijs/

Library to communicate with Apple iOS devices

http://www.libimobiledevice.org

Kevent based inotify compatible library

https://github.com/libinotify-kqueue/libinotify-kqueue

Generic input library

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

ODBC 3.x driver manager, for universal data source access

http://www.iodbc.org/

Library for manipulating IPTC metadata stored in image files

http://libiptcdata.sourceforge.net/

Library for Evation's Irman infrared reciever

http://www.evation.com/libirman/

A stand-alone client/server 9P library

http://www.suckless.org/

Codec for karaoke and text encapsulation for Ogg

http://wiki.xiph.org/index.php/OggKate

KSBA is an X.509 Library

http://www.gnupg.org/aegypten2/

Interface library to access tags for identifying languages

http://tagoh.bitbucket.org/liblangtag/

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.info/

Provides a few data structures and helpful functions

https://github.com/cheusov/libmaa

Libmad library (part of MAD project)

http://mad.sourceforge.net/

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

The C library for accessing a memcache cluster

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

Set of utilities to benchmark productivity of system calls

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

MikMod Sound Library

http://mikmod.sourceforge.net/

ModPlug mod-like music shared libraries

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

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/

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

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

Library and transmitter that implements ICE-19

http://nice.freedesktop.org/

Library and transmitter that implements ICE-19

http://nice.freedesktop.org/

Library and transmitter that implements ICE-19

http://nice.freedesktop.org/

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

http://libnjb.sourceforge.net/

Library for desktop notifications

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

Astronomical Calculation Library

http://libnova.sourceforge.net/

Library that implement Microsoft's NTLM authentication

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

C library implementing the OAuth Core standard

http://liboauth.sourceforge.net/

Replacement Objective-C runtime supporting Obj-C 2 features