Library for fast Base64 encoding and decoding

https://github.com/stefantalpalaru/libb64

Prusa Block & Binary G-code reader/writer/converter

https://github.com/prusa3d/libbgcode

Blu-Ray discs playback library for media players

https://www.videolan.org/developers/libbluray.html

C library implementing the Cangjie input method

https://github.com/Cangjians/libcangjie

Library for collision detection between two convex shapes

https://github.com/danfis/libccd

Library and tools for parsing Corel Draw file format

https://wiki.documentfoundation.org/DLP/Libraries/libcdr

File metadata and streaming library

https://github.com/Ultimaker/libCharon

Chewing intelligent phonetic input method library

https://chewing.im/ https://github.com/chewing/libchewing

Ultralightweight JSON parser in ANSI C

https://github.com/DaveGamble/cJSON

Client library for the CMIS interface

https://github.com/tdf/libcmis

Library for accessing the MusicBrainz cover art archive

https://coverartarchive.org/

CSS2 parsing library

http://www.freespiders.org/projects/libcroco/

Minimal and cross platform C++ GUID library

https://github.com/graeme-hill/crossguid

GNOME application for managing encryption keys (PGP, SSH)

http://seahorse.sourceforge.net/

Cascading Style Sheets library

https://www.netsurf-browser.org/projects/libcss/

Performant hash table implementation

https://github.com/efficient/libcuckoo

CUE Sheet Parser Library

http://sourceforge.net/projects/libcue/

C++ utility library with generic and independent components

https://www.codesynthesis.com/projects/libcutl/

WebRTC Data Channels, WebRTC Media Transport, and WebSockets library

https://github.com/paullouisageneau/libdatachannel

Free DTS Coherent Acoustics decoder

http://www.videolan.org/developers/libdca.html

Open source h.265 video codec

https://www.libde265.org

Fast, whole-buffer DEFLATE-based compression library

https://github.com/ebiggers/libdeflate

Generic Device Query and Monitor interface

https://github.com/freebsd/libdevq

Grand Central Dispatch API support library

http://libdispatch.macosforge.org/

EDID and DisplayID library

https://gitlab.freedesktop.org/emersion/libdisplay-info

Userspace interface to kernel Direct Rendering Module services

http://dri.freedesktop.org

Quasar DV codec (libdv): software codec for DV video encoding format

http://libdv.sourceforge.net/

MPlayer version of the libdvdnav project

http://dvdnav.mplayerhq.hu

MPlayer version of the libdvdread project

http://dvdnav.mplayerhq.hu

EBML (Extensible Binary Meta Language), sort of binary version of XML

https://www.matroska.org/

Implementation of the EBU R128 loudness standard

https://github.com/jiixyj/libebur128

Command line editor library

http://thrysoee.dk/editline/

Library for Emulated Input

https://gitlab.freedesktop.org/libinput/libei

epoll shim implemented using kevent

Liberation fonts from Red Hat to replace MS TTF fonts

https://fedorahosted.org/liberation-fonts/

C++11 simple, header-only, TIFF reader

https://github.com/libertiff/libertiff

Library for posting electronic mail

http://www.stafford.uklinux.net/libesmtp/

Library for some string essentials

https://libestr.adiscon.com/

Library to interpret and import Apple Keynote presentations

https://wiki.documentfoundation.org/DLP/Libraries/libetonyek

Mail framework for C language

http://www.etpan.org/libetpan.html

API for executing callback functions on events or timeouts

http://libevent.org/

Library to read digital camera file meta-data

http://sourceforge.net/projects/libexif/

Application library for the Xfce desktop environment

http://www.xfce.org/

Language guessing by N-Gram-Based Text Categorization

https://wiki.documentfoundation.org/Libexttextcat

Open Fabric Interfaces

https://ofiwg.github.io/libfabric/

X virtual keyboard library

https://www.yoctoproject.org/tools-resources/projects/matchbox

Fork of json-c (JSON implementation in C)

https://github.com/rsyslog/libfastjson

Foreign Function Interface

http://sources.redhat.com/libffi/

Provides library functionality for FIDO 2.0

https://developers.yubico.com/libfido2/ https://github.com/Yubico/libfido2

Formatting library for C++

https://fmt.dev/