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/

Online presence and instant messaging server

https://jabberd2.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

High performance image processing libraries for Java

http://java.sun.com/products/java-media/jai/

A library for generating Java bytecode

http://jakarta.apache.org/bcel/

Java library for command line arguments and options

http://jakarta.apache.org/commons/cli/

Implementations of common encoders and decoders

http://jakarta.apache.org/commons/codec/

Package implementing the client side of the HTTP standard

http://jakarta.apache.org/commons/httpclient/

Generic logging wrapper library

http://jakarta.apache.org/commons/logging/

A library supporting Java primitive types

http://jakarta.apache.org/commons/primitives

Perl/awk/sh regular expressions for Java

https://jakarta.apache.org/oro/

Regular expressions for Java

http://jakarta.apache.org/regexp/

Apache Struts framework

http://struts.apache.org

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

Just Journal for Java blogging app

https://www.justjournal.com/

Reference Implementation of JSR 919

https://javaee.github.io/javamail/

Wrapper script for various Java Virtual Machines

Java XPath Engine

https://jaxen.org

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/

JDBC connection pool

http://www.bitmechanic.com/projects/jdbcpool/

Java library for accessing and manipulating XML documents

http://www.jdom.org/

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/

Converts JFile database to CSV, and vice versa

http://www.land-j.com/jfileu.html

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/

Dynamic web content management system (CMS)

http://www.joomla.org/

IJG's jpeg compression utilities

http://www.ijg.org/

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/