Implementation of the IETF Calendaring and Scheduling protocols

https://github.com/libical

Character set conversion library

https://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

Library for Evation's Irman infrared receiver

https://libirman.sourceforge.io/

Standalone client/server 9P library

https://libs.suckless.org/deprecated/libixp/

Codec for karaoke and text encapsulation for Ogg

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

Library to make X.509 certificates

https://www.gnupg.org/aegypten2/

Lightweight Open Sound Control implementation

http://liblo.sourceforge.net/

Easy to use C/C++ seam carving library

http://liblqr.wikidot.com/

System independent dlopen wrapper

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

Libmad library (part of MAD project)

https://www.underbit.com/products/mad/

A library encapsulating malsync

http://jasonday.home.att.net/code/libmal/

C library for accessing a memcache cluster

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

Set of utilities to benchmark productivity of system calls

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

Library for Microsoft compression formats

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

MusicBrainz client library for audio metadata lookup

http://musicbrainz.org/

C library for creating IP packets

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

Ogg bitstream library

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

Library for handling OpenType fonts

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

Library providing routines for paper size management

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

Ubiquitous network traffic capture library

https://www.tcpdump.org/

PCI configuration space I/O made easy

https://mj.ucw.cz/sw/pciutils/

Generic PCI access library

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

Library to handle Apple Property List format

http://www.libimobiledevice.org

Library that provides automatic proxy configuration management

https://libproxy.github.io/libproxy/

Weak aliases for pthread functions

https://xcb.freedesktop.org/

RTMP stream library

https://rtmpdump.mplayerhq.hu/

Handling page faults in user mode

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

Reading and writing files containing sampled sound (like WAV or AIFF)

http://www.mega-nerd.com/libsndfile/

Library implementing the SSH2 protocol

https://www.libssh.org/

Library implementing the SSH2 protocol

https://www.libssh2.org/

Strigi desktop search: streams library

http://www.vandenoever.info/software/strigi/

Wrapper providing SunOS NFSv4 ACL API

Parser for SVG content in files or buffers

http://cairographics.org/libsvg

ASN.1 structure parser library

https://www.gnu.org/software/libtasn1/

Generic shared library support script

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

User space caching library

http://mercurial.creo.hu/repos/libublio

Library of Unicode annotation data

http://libuninameslist.sourceforge.net/

Library to work with RAR archivies

http://www.rarsoft.com/

Video4Linux library

https://linuxtv.org/wiki/index.php/V4l-utils

Provide an easy API to a custom vnc server

http://libvnc.github.io/

Library to provide file system type information

Audio compression codec library

https://xiph.org/vorbis/

The X protocol C-language Binding (XCB) library

https://xcb.freedesktop.org/

Implementation of the XDG Base Directory specification

https://github.com/devnev/libxdg-basedir/

Extension library for the Xfce desktop environment

https://www.xfce.org/

XKB file library

https://www.freedesktop.org/Software/xorg

The xkbui library

http://www.freedesktop.org/Software/xorg

Utility library to make XKB stuff easier

http://gswitchit.sourceforge.net/