The Ruby language has a very robust and high-performance implementation on the JVM - JRuby. While JRuby by itself brings a lot of (implicit) benefits of the JVM to the Ruby world, the rest-of-the-world isn't without its share of excitement!
Scala, Clojure, Mirah et al have a host of features which make them very expressive and powerful for certain classes of problems. For example, type-safety combined with the compilation phase bring in performance improvements. Or immutability allows for easy-to-reason-about concurrent implementations. Lazy evaluation allows for easier expression and solution of still another class of problems. And for situations requiring you to handle asynchronous situations, there are Actor libraries.
With JRuby, there are two ways to leverage the benefits of these languages. Since everything boils down to classes (and typically, JAR-libraries), one can directly call into these libra