The latest MacRuby with HotCocoa is amazing! Finally a way to write OS X applications without having to learn Objective-C.
require 'rubygems'
require 'hotcocoa'
include HotCocoa
application do |app|
win = window :size => [100,50]
b = button :title => 'Hello'
b.on_action { puts 'World!' }
win << b
end