How to build OS X GUI applications with Ruby and RubyCocoa
Erik Kastner has put together a great tutorial showing you how to build a graphical OS X app using Ruby and RubyCocoa. For a less wordy approach, there's also a short screencast which shows you the whole process from start to finish.
With the continual improvements of the RubyCocoa library, and Apple's commitment to Ruby for Cocoa development in OS X 10.5, Ruby is rapidly becoming a viable alternative to Objective C and Java for developing OS X apps.
Another great resource is RubyCocoa.com, a site that features several RubyCocoa tutorials and articles.