Check out the Ruby Quiz run by James Edward Gray II. It's a weekly 'quiz' that runs on the ruby-talk mailing list each week. I think this is a great idea for any language, but especially emerging languages full of interesting features like Ruby.
This week's quiz was a meta-programming challenge called metakoans. This was a really great quiz highlighting a less common, but incredibly powerful feature available in Ruby (which borrowed much of this from Lisp).