IntelliJ Platform is an OSS platform developed by JetBrains for building IDEs and language-aware developer tools. It is used by IntelliJ IDEA, Android Studio, CUBA Studio and Cursive, to name a few. Built on Java, it provides a cross-platform approach to building tools for any language, whether targeting the JVM or not. It is available under an Apache 2.0 license allowing you to create royalty-free commercial and non-commercial products on it, including fully-fledged IDEs and plugins.
The IntelliJ Platform has a lot of overlap with the IntelliJ IDEA Community Edition, the free and Open Source edition of IntelliJ IDEA. The source code for both projects is hosted at GitHub. The core components include a Virtual File System, a Text Editor, a UI framework, a Debugger, and a Test Runner. The platform also provides APIs for building support for custom languages and frameworks, including syntax highlighting, navigation, code completion, inspections, intentions, refactorings, a debugger, test runners, tool windows, actions, and more, as well as general lexers and parsers.
JetBrains hosts a Plugin Marketplace where any plugin can be published and installed on any IDE built on the platform. More information about the IntelliJ Platform is available in the Developer Documentation.