Kotlin Open Sourced
The basic compiler (which they call kompiler, starting a new legacy of K-prefixed terms – even kontributors, which they are looking for …) is available separately and can be embedded in Maven, Ant or Gradle toolchains. This allows code to be developed in the IDE to be built with existing mechanisms, requiring minimal intervention in order to use in a new environment, or even if working on a project with other developers who do not have the Kotlin plugin installed.
The IntelliJ Kotlin plugin extends the Java compiler to allow Kotlin code to be written, compiled and debugged. In addition, helper functions have been written for the basic Java collections, which allow a more fluid interface ahead of the Collections extensions coming up in Java 8.
There are separate Kotlin to Java comparison and Kotlin to Scala comparison papers available, compare and contrasting the features of both. Whilst Scala may remain more powerful, Kotlin tries to be a better Java with functions, mattern matching, null pointer avoidance and generics. The language also supports traits and pattern matching.
The Kotlin plugin will be available in a future release of IntelliJ (though the Early Access Program has preview builds available, and snapshot builds of Kotlin are available via the JetBrains GitHub page). However, you can get a feel for the language now with a web-based demo which allows you to edit Kotlin code, compile and run via your web browser.
Kotlin, like Xtend, is aiming to be a better Java rather than an entirely new platform. Both compile down to bytecode (although Xtend translates into Java source first and lets the Java compiler do the hard work), and both bring functions and extension functions (the ability to statically add a new method to an existing type within a limited scope). Whilst Xtend is Eclipse-based, and Kotlin is IntelliJ based, both have headless builds. The winner might just be the language which evolves to the other IDE first.