As it turns out, I've already got Ruby installed on my iBook G4 since I'm running OS X 10.4. If you are on Windows or some other machine that does not have Ruby already installed, you can get it at http://www.ruby-lang.org, where they also have plenty of instructional information.
So far, I've only read the first chapter and have done the requisite "Hello, World!" program:
puts "Hello, World!"
Pretty simple, huh? Sure beats the pants off of Java for doing simple IO. As a software tester by profession, I think Ruby will be handy in my toolbag. And since it has gotten quite popular, I might as well try it as an alternative to Perl for my scripting and testing needs.