If you know me, you know that I love Ruby. It just makes programming more fun and is a phenomenal tool for effectively developing software in a powerfully succint way.
As part of a broader plan for world domination, I've always been interesting in seeing Ruby run on .NET's CLR or Java's JVM. Fortunately, there's a lot of work taking place with the RubyCLR project and JRuby.
Regarding JRuby specifically, there is an incredible amount of work taking place. I'm just starting to play around with it, but I've already provided some suggestions that the very competent JRuby team has simply pounced on.
The promise of marrying the power of Ruby with the ubiquity of Java and the capability of Swing certainly presents some very interesting possibilities. Here's a quick example of JRuby in action. For reference, here's the same code in Java.