Why would any self-respecting Java developer care about Ruby? Ruby is a general-purpose scripting language created 10 years ago in Japan. Contrary to popular belief, it is a pure object-oriented language. Unlike Java technology, Ruby has no scalars, so everything, including integers, are first-class objects. Ruby's syntax borrows heavily from Smalltalk, Python, and Ada. Like the Java programming language, Ruby is a single inheritance language, but it offers some advanced features that Java technology does not, such as closures (think anonymous inner classes on steroids) and mix-ins (similar to interfaces, but less tightly bound to the class). Ruby is also highly portable, running on all major operating systems.
Ruby is also red-hot right now. People are starting to use it for the types of applications where it excels. Because it is interpreted and uses dynamic typing, you can do all sorts of magic tricks at run time that are very difficult in Java. One of the surprising capabilities enabled by dynamic typing and expressive syntax is the ability to create domain-specific languages in Ruby that allow you to work at a higher level of abstraction, away from the "raw" syntax of the language. Ruby on Rails is a framework for creating Web applications backed by databases that shows this elegance. Rake, Ruby's version of Make and Ant rolled into one, is another example of this powerful use of the language.
The other reason to start playing with Ruby is that all the cool kids are doing it. Many of the people who recognized that Java technology was going to be important back in 1996 (like Glenn Vanderburg, Bruce Tate, and Martin Fowler) are heavily involved in Ruby now. Even if you aren't going to change all your development efforts to Ruby, maybe it's time you took a look at this language.
One of the main limiting factors to widespread development in Ruby is a good development environment (for those who don't want to learn Emacs). The RDT changes that. What better way to experiment with Ruby than in your favorite IDE, Eclipse?