JRuby: An Introduction is a fresh new article by Chris Duckett that presents a practical, code-driven introduction to JRuby, a Java implementation of Ruby. It's the perfect introduction for people who, perhaps, have an idea of what JRuby is, but who haven't yet tried out any of the Java connectivity. The first page looks at interacting with basic Java libraries / classes, with the second moving on to building a basic Swing-powered GUI app using Java APIs from Ruby code.

Added: There's also a screencast by Brian Chapados called Intro to JRuby that's worth watching for a more practical, real-time look.