Running Ruby in the Browser is a proof of concept demo of embedding Ruby in a web browser using JRuby:

This is an experiment to run Ruby in the browser. It uses an embedded JRuby applet, and the page communicates to that puppy via LiveConnect. This simple example shows running code via script type="text/jruby" (which is run automatically once, which you see in the output below), and from a form itself. From here we will allow smart bidirectional talk between the DOM and Ruby, and offer up a nice abstraction layer to hopefully enable something like:

<script type="text/ruby">
  document.ready do |dom|
    dom["table tr"] <<"<td>test</td>"
  end
</script>