Multi-purpose desktop calculator (backend library)

http://qalculate.github.io/

C library for encoding data in a QR Code symbol

http://megaui.net/fukuchi/works/qrencode/index.en.html

Secret Rabbit Code: a Sample Rate Converter for audio

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

Callback Framework for C++

http://libsigc.sourceforge.net/

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/

Library permits a programmer to develop software

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/

Sender Policy Framework (SPF) library and client

http://software.schmorp.de/pkg/libspf.html

Sender Rewriting Scheme 2 C Implementation

https://www.libspf2.org/

Library to convert clothoid splines into Bezier splines

https://github.com/fontforge/libspiro

Library implementing the SSH2 protocol

https://www.libssh.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/

Theora video codec for the Ogg multimedia streaming system

https://www.theora.org/

TI calculator file types library

http://tilp.info/

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

Tools and library routines for working with GIF images

http://sourceforge.net/projects/libungif/

Library for manipulating Unicode characters and strings

https://www.gnome.org/

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

VDPAU wrapper and tracing library

http://freedesktop.org/wiki/Software/VDPAU

Abstraction library that sits between apps and visual plugin

http://libvisual.sourceforge.net/

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/

Tools and library for converting Microsoft WMF (windows metafile)

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

The W3C Reference Library

http://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 parser library for GNOME

http://xmlsoft.org/

XML API for C++

http://libxmlplusplus.sourceforge.net/

XML API for C++

http://libxmlplusplus.sourceforge.net/

XML parser library for GNOME

http://xmlsoft.org/

The XSLT C library for GNOME

http://xmlsoft.org/XSLT/

YAML 1.1 parser and emitter written in C

https://pyyaml.org/wiki/LibYAML

C library for reading, creating, and modifying ZIP archives

https://libzip.org/

Routines to access raw VBI capture devices

http://zapping.sourceforge.net/doc/libzvbi/