I hope you like my journalistic headline.
Anyway Watir (pronounced water) and watir-webdriver are two ruby gems that allow you to test web apps..
From the watir website
Watir, pronounced water, is an open-source (BSD) family of Ruby libraries for automating web browsers. It allows you to write tests that are easy to read and maintain. It is simple and flexible.
The whole set of software allows you to programmatically test web applications in all 4 major browsers - IE, Firefox, Chrome and Safari. You can interact with the DOM and check for the correct changes allowing testing of page interaction and AJAX functions.
A simple program looks something like this..
browser.text_field(:name => "q").set "watir"
browser.div(:id => "resultStats").wait_until_present
browser.title.should == "watir - Google Search"
Nice and simple!