Open-source plugins included in the distribution of IntelliJ IDEA Ultimate and other IDEs based on the IntelliJ Platform

CheckStyle plug-in for IntelliJ IDEA

The fast scanner generator for Java™ with full Unicode support

Laravel Framework Plugin for PhpStorm / IntelliJ IDEA

IntelliJ plugin - use Eclipse's code formatter directly from IntelliJ. https://plugins.jetbrains.com/plugin/6546

Haxe plugin for IntelliJ Platform based IDEs (IDEA, Android-Studio)

Wolfram Language and Mathematica plugin for IntelliJ IDEA.

Handlebars and Mustache template support for Intellij IDEA platform IDEs

IntelliJ IDEA plug-in for Visual Studio Team Services and Team Foundation Server (TFS)

Code to brute-force a specific incomplete QR code

Native Terminal Plugin for JetBrains IDEs

IntelliJ plugin for Scheme programming language

(Intellij) decompiles jar library and attaches sources to current project

Jumping beyond Java...

IntelliJ plugin to support the Arc programming language