"Web Application Testing In Ruby" - http://wtr.rubyforge.org/

Uses the win32ole module to drive IE's COM interface. Spectacularly stable and deterministic compared with, say, Selenium. 

If you're Linux/Osx based WaTir works wonderfully well over the WineCompatibilityLayer.

If you are anything-based you will also enjoy FireWater. It uses a MozillaFirefox plugin with a back-door to access your live JavaScript

''after you figure out how to initialize the browser! we had to go with...''

Rakefile:
  task :default do
    system 'killall firefox-bin'
    sh '/Applications/Firefox.app/Contents/MacOS/firefox -jssh &'
    sh 'ruby automated_test.rb'
  end

acceptance_test.rb:
  require 'firewatir'
  include Fire''''''Watir
                                         
  $browser = Firefox.new(:port => 9997, :profile => 'firewatir')
...
  class AcceptanceTest < Test::Unit::TestCase
    def setup
      @browser = $browser
    end
  ...
  end

''looks like a job for ConfigurationHell! :-)''