Akephalos was a great little Capybara driver that wrapped HtmlUnit to provide a JavaScript-aware, headless browser for full-stack acceptance testing which was an order of magnitude faster than the then-incumbent king, Selenium.

Unfortunately, Akephalos now looks to be abandonware. Luckily, even nicer things have sprung up in its wake, the best of which currently appears to be capybara-webkit, which is more than twice as fast in my testing and just as reliable.

Official site