Doppio is a JVM implementation in Coffeescript. I worked on it with CJ Carey and Jonny Leahey as part of a class this semester, but we took it a lot further than was required – it now runs Rhino as well as the Java 4 compiler. The about page describes some of the architecture.
I hadn’t quite planned to announce the project yet, but it found its way onto reddit and hackernews, so I guess the cat’s out of the bag. Pardon the rough edges!