Mirah (nee Duby) is a new experimental language born out of the JRuby project. In order to make implementing Ruby on the JVM easier and more approachable for Java and Ruby developers alike.
Mirah’s design is centered around a few simple principals: Platform-agnostic, Free from concrete decisions about the back-end type system, code generation, or other details are specified by the outward language. This means roughly similar scripts could conceivably compile to any number of type systems and runtimes. In this sense, Mirah is more of a rough coupling of Ruby-like syntax with a pluggable type-inference and compilation pipeline.
Tuga is a small, opinionated, statically-typed scripting language that is oriented around OOP and getting things done. It has some aspects of Ruby, Python, Java, and ECMAScript but aims to be better than all these at building large-scale systems while still being good for quick scripts.
JRuby is the effort to recreate the Ruby interpreter in Java. Currently, the interpreter is written in C. The Java version will be tightly integrated with Java to allow both to script any Java class and to embed the interpreter into any Java application.