I'm one of the authors. Doppio's not the first JVM in Javascript, but I believe we have the most extensive support for the JRE among the lot. In particular, we can run Rhino and the Java 4 compiler. Feel free to ask me any questions! Check out the demo here: http://int3.github.com/doppio/
Rhino on Doppoio on JavaScript has got to be the slowest thing in the world.
I can't upvote this comment, but JVM is the greatest of all time.