Events
There's not much to say about Events. It does what it says and the documentation for its entire functionality fits into one short README.
It transparently uses Kotlin coroutines and is compatible with the JVM and recently I even used it in an Android app. By transparently I mean that you can control your required concurrency when emitting events using the facilities provided by the Kotlinx Coroutines library.
Also one of my projects I haven't touched in years, it just works. Why can't all things be as simple?