Open-source continuous integration server

https://jenkins.io/

JetBrains CLion IDE

https://www.jetbrains.com/clion/

Free and open source typeface for developers

https://www.jetbrains.com/lp/mono

jf Open Huninn by justfont

https://justfont.com/huninn/

Simulates a jigsaw puzzle and illustrates the use of clipping paths

http://gap.nongnu.org/jigsaw/

Java source to byte-code compiler made available by IBM

http://jikes.sf.net/

Command line client for posting JustJournal entries

https://www.justjournal.com/

Jerry's command-line LiveJournal.com entry system

http://umlautllama.com/projects/perl/#jlj

Jim Knoble's font package for X

http://www.jmknoble.net/fonts/

Java access to native shared libraries

https://github.com/java-native-access/jna

Joe's Own Editor

https://sourceforge.net/projects/joe-editor/

Dynamic web content management system (CMS)

https://www.joomla.org/

Dynamic web content management system (CMS)

https://www.joomla.org/

SIMD-accelerated JPEG codec which replaces libjpeg

https://libjpeg-turbo.org/

Lightweight and flexible command-line JSON processor

https://jqlang.github.io/jq/

View and/or control a remote PC

https://jrdesktop.sourceforge.net/

JSON (JavaScript Object Notation) implementation in C

https://github.com/json-c/json-c/wiki

JSON (RFC 4627) interface for Glib

http://live.gnome.org/JsonGlib

JSON reader and writer library for C++

https://github.com/open-source-parsers/jsoncpp

General purpose dynamic array

https://judy.sourceforge.net/

Humanist sans-serif font

https://www.theleagueofmoveabletype.com/junction

Unicode/MUFI OpenType font for medievalists (Latin, IPA, Runic, Greek)

http://junicode.sourceforge.net/

Regression testing utility for use with the Java(TM) Language

https://www.junit.org/

The editor which sucks less

http://www.mirbsd.org/jupp.htm

Kubernetes command-line tool

https://github.com/derailed/k9s

Makefile framework

https://svn.netlabs.org/kbuild

Free Unicode font for Sinhala script

http://www.kaputa.com/slword/kaputaunicode.htm

Light-weight and easy-to-use password manager

https://keepass.info/

KeePass plugin which facilitates the Kee Firefox plugin

https://www.kee.pm/

Portable scriptable network and serial communication program

https://www.kermitproject.org/

Library for registering keyboard shortcuts

https://github.com/engla/keybinder

Library for registering keyboard shortcuts

https://github.com/engla/keybinder

Extra modules and scripts for CMake

https://quickgit.kde.org/?p=extra-cmake-modules.git

Khmer OS fonts

https://sourceforge.net/projects/khmer/

Browser based analytics and search interface to ElasticSearch

https://www.elastic.co/products/kibana

Compiler of Keyman KMFL keyboard sources to use with SCIM/IBus

https://kmfl.sourceforge.net/

Example KMS/GBM/EGL application

https://gitlab.freedesktop.org/mesa/kmscube

High performance authoritative-only DNS server

https://www.knot-dns.cz/

High performance authoritative-only DNS server

https://www.knot-dns.cz/

High performance authoritative-only DNS server

https://www.knot-dns.cz/

Web application framework for writing web APIs in C

http://www.kore.io/

MIT implementation of RFC 4120 network authentication service

https://web.mit.edu/kerberos/

Official AT&T release of KornShell 93

http://www.kornshell.com/

Development branch of AT&T KornShell 93

http://www.kornshell.com/

Testing framework for infrastructure software

https://github.com/jmmv/kyua/

MP4 muxer and other tools

https://l-smash.github.io/l-smash/

Cracks SAMBA or Windows NT passwords

Icon pack designed to integrate with most desktop environments

https://krourke.org/projects/art/la-capitaine-icon-theme

Linux Audio Developer's Simple Plugin API

https://www.ladspa.org/

Fast MP3 encoder kit

http://lame.sourceforge.net/