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, Cursive to name a few. Built on Java, it provides a cross-platform approach to building tools for any kind of language, independently of whether this targets the JVM or not. It is available under Apache 2.0 license allowing you build royalty-free commercial and noncommercial products on it, whether these are fully fledged IDEs or plugins.
The IntelliJ Platform source code is hosted at GitHub. The core components include a Virtual File System, Text Editor, UI framework Debugger, Test Runner. The platform also provides APIs for building custom language and framework support: navigation, code completion, inspections, intentions, refactorings, debugger, test runners, tool windows, actions, etc. as well as general lexers and parsers.
JetBrains hosts a plugin repository 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.