Watir is by far my preferred tool for testing web applications.
Watir is… An open source Ruby library for automating tests. Watir interacts with a browser the same way people do: clicking links, filling out forms and validating text.
Watir-webdriver is a modern version of the Watir API based on Selenium. Selenium 2.0 (selenium-webdriver) aims to be the reference implementation of the WebDriver specification. In Ruby, Jari Bakken has implemented the Watir API as a wrapper around the Selenium 2.0 API. Not only is Watir-webdriver derived from Selenium 2.0, it is also built from the HTML specification, so Watir-webdriver should always be compatible with existing W3C specifications.
Watir is available as a Ruby gem.
And of course because it’s Ruby, it’s fun!