app.rb

%w[ rubygems sinatra haml ].each {|lib| require lib }

get '/' do
  "Hello World!  The time is now #{ Time.now }"
end

app_spec.rb

%w[ rubygems spec capybara capybara/dsl ].each {|lib| require lib }
require File.dirname(__FILE__) + '/app'
require File.dirname(__FILE__) + '/capybara_webdriver'

include Capybara
Capybara.app = Sinatra::Application
Capybara.current_driver = :selenium

use_selenium_driver :htmlunit

describe 'App' do
  it 'should test stuff' do
    100.times do
      visit '/'
      page.should have_content('Hello World')
    end
  end
end

capybara_webdriver.rb

def use_selenium_driver driver
  $__capybara_selenium_driver_name = driver
end

def selenium_driver
  $__capybara_selenium_driver_name || :htmlunit
end

def (Capybara::Driver::Selenium).driver
  unless @driver
    if selenium_driver == :htmlunit
      @driver = Selenium::WebDriver.for :remote, 
        :url                  => 'http://localhost:4444/wd/hub', 
        :desired_capabilities => Selenium::WebDriver::Remote::Capabilities.htmlunit
    else
      @driver = Selenium::WebDriver.for selenium_driver
    end
    at_exit do
      @driver.quit
    end 
  end 
  @driver
end

run_webdriver_server_for_htmlunit.sh

java -jar selenium-server-standalone-2.0a2.jar