Windmill is kind of like Selenium in that it uses javascript to test web applications, but it is also powered by python which is very cool. It is part of the Chandler project, which I think is an offshoot of the Mozilla Firefox project.
I was able to spend some time playing around with it, and it works pretty well. It reminds me a lot like Selenium, and the authors say they wrote it because there were some things they couldn't do in Selenium that they needed. I'm curious what those things are, and why they didn't just extend Selenium.