Tool to measure maximum TCP and UDP bandwidth

https://sourceforge.net/projects/iperf2/

KAME racoon IKE daemon, ipsec-tools version

http://ipsec-tools.sourceforge.net/

High performance realtime 3D engine

http://irrlicht.sourceforge.net/

Modular IRC client with many features

https://irssi.org/

Collection of useful scripts for the irssi IRC-client

https://scripts.irssi.org/

Collection of additional irssi themes

https://irssi-import.github.io/themes/

ISC Cron, former Vixie Cron

ftp://ftp.isc.org/isc/cron/

Lists of the country, language, and currency iso names

https://salsa.debian.org/iso-codes-team/iso-codes

Character entity sets from ISO 8879:1986 (SGML)

Make XML documents translatable through po files

https://itstool.org/

Low latency audio server

https://jackaudio.org/

Object-oriented SGML/XML parser toolkit and DSSSL engine

http://www.jclark.com/jade/

JAGS - Just Another GTK+ Samba Client

http://jags.sourceforge.net

Setuid version of jexec to allow normal users access to jails

https://github.com/Intermedix/jailme

Wrapper code to start/stop a Java application as a daemon

http://commons.apache.org/daemon/

Build utility like make(1)

https://www.perforce.com/jam/jam.html

Compact Java virtual machine

https://jamvm.sourceforge.net/

C library for encoding, decoding, and manipulating JSON data

https://github.com/akheron/jansson

Implementation of the codec specified in the JPEG-2000 standard

https://www.ece.uvic.ca/~frodo/jasper/

Updated Java timezone definitions

Wrapper script for various Java Virtual Machines

Decoder implementation of the JBIG2 image compression format

https://github.com/ArtifexSoftware/jbig2dec

Lossless compression for bi-level images such as scanned pages, faxes

https://www.cl.cam.ac.uk/~mgk25/jbigkit/

Mainline variant of Kademlia Distributed Hash Table (DHT)

http://www.pps.univ-paris-diderot.fr/~jch/software/bittorrent/

Open-source continuous integration server

https://jenkins.io/

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/

English to Japanese dictionary for GNUstep

https://www.eskimo.com/~pburns/Dictionaries/

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/

Joe's Own Editor

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

IJG's jpeg compression utilities

http://www.ijg.org/

SIMD-accelerated JPEG codec which replaces libjpeg

https://libjpeg-turbo.org/

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/

The editor which sucks less

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

Portable scriptable network and serial communication program

https://www.kermitproject.org/

Browser based analytics and search interface to ElasticSearch

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

Web application framework for writing web APIs in C

http://www.kore.io/

Testing framework for infrastructure software

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

Cracks SAMBA or Windows NT passwords

Linux Audio Developer's Simple Plugin API

https://www.ladspa.org/

Fast MP3 encoder kit

http://lame.sourceforge.net/

Linear Algebra PACKage

https://www.netlib.org/lapack/

Tetris-like game for GNUstep

http://www.nongnu.org/gap/lapispuzzle/

Session management system for JACK audio applications

https://www.nongnu.org/lash/

Image viewer and slideshow application

http://gap.nongnu.org/