Multi-purpose desktop calculator (backend library)

http://qalculate.github.io/

Free version of the SSL/TLS protocol forked from OpenSSL

https://www.libressl.org/

Easy access to RESTful web services

https://wiki.gnome.org/Projects/Librest

RTMP stream library

https://rtmpdump.mplayerhq.hu/

Secret Rabbit Code: a Sample Rate Converter for audio

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

Library to access the secret service API

https://wiki.gnome.org/Projects/Libsecret/

Callback Framework for C++

http://libsigc.sourceforge.net/

Callback Framework for C++

http://libsigc.sourceforge.net/

Handling page faults in user mode

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

Routines for rapid alpha-numeric terminal applications development

https://www.jedsoft.org/slang/

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

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

SOAP (Simple Object Access Protocol) implementation in C

https://libsoup.org/

SOAP (Simple Object Access Protocol) implementation in C

Sender Policy Framework 2 C Implementation

https://www.libspf2.org/ https://github.com/shevek/libspf2

Secure RTP (SRTP) Reference Implementation

https://github.com/cisco/libsrtp

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/

Thai language support library

https://linux.thai.net/projects/libthai/

Theora video codec for the Ogg multimedia streaming system

https://www.theora.org/

Generic shared library support script

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

BitTorrent Library

https://github.com/rakshasa/libtorrent

Lightweight fully POSIX compliant regexp matching library

https://laurikari.net/tre/

User space caching library

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

Library of Unicode annotation data

https://github.com/fontforge/libuninameslist

Library for converting Unicode strings to numbers

http://billposer.org/Software/libuninum.html

Unicode string library

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

RAR archive extraction library

https://www.rarlab.com/

Communication interface library for usbmuxd

https://www.libimobiledevice.org

Video4Linux library

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

Abstraction library that sits between apps and visual plugin

https://libvisual.sourceforge.net/

Provide an easy API to a custom vnc server

https://libvnc.github.io/

Library to provide file system type information

Audio compression codec library

https://xiph.org/vorbis/

VP8/VP9 reference encoder/decoder

https://www.webmproject.org/

Tools and library for converting Microsoft WMF (windows metafile)

https://wvware.sourceforge.net/libwmf.html

Tools for importing and exporting WordPerfect(tm) documents

http://libwpd.sourceforge.net/

W3C Reference Library

https://www.w3.org/Library/

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/

XML API for C++

http://libxmlplusplus.sourceforge.net/

XML parser library for GNOME

http://xmlsoft.org/

Shared memory 'SyncFence' synchronization primitive

https://gitlab.freedesktop.org/xorg/lib/libxshmfence