Letter: J
- jam - Program construction tool, similar to make
- jasper - Implementation of the JPEG-2000 standard, Part 1
- jhead - Tool for displaying EXIF data embedded in JPEG images
- jikes - A Java source file to bytecode compiler
- jlint - Java program checker
- john - John the Ripper password cracker
- jpgraph - An OO graph drawing class library for PHP
- js - JavaScript interpreter and libraries