For a little side project I am currently evaluating different ways of GUI programming using Ruby. Monkeybars is one of the most interesting candidates. There is a really good article about it, if you want to learn more.
Version 1.0.4 has a little, but annoying bug: If you generate a new application skeleton it won’t compile/run:

manifest.rb:32:in `require': no such file to load -- monkeybars (LoadError)
	from manifest.rb:32
	from manifest.rb:21:in `require'
	from main.rb:21

To fix it, you got to change line 21 of manifest.rb:

add_to_classpath '../lib/java/monkeybars-1.0.2.jar'
#to
add_to_classpath '../lib/java/monkeybars-1.0.4.jar'