Watir (Web Application Testing in Ruby) implementation built on WebDriver's Ruby bindings. Used for web browser automation, this tool allows you to code in Ruby using the friendly Watir API but get the cross browser support of Webdriver to automate Chrome, Firefox, IE, Opera or a 'headless' browser. Distributed as a Ruby gem named 'watir-webdriver'
For those who don’t know what Watir-WebDriver is, it’s basically a nice Watir (ruby) implementation on WebDriver, so it gives you five browsers (four real, one headless) using one neat API, out of the box.
The Watir API uses an object model that parallels the HTML element model and the browser DOM. This allows you to write very natural OOP code where you can have a button in the UI click itself. (as opposed to Selenium where the 'browser' object does everything, and takes the elements as parameters for methods like click.
See: watirwebdriver.com