Selenium is a Firefox extension that basically allows you to record some actions in Firefox and then Assert some things. You can add breakpoints and save the tests and edit them using any HTML editor because it is all XML.
I can see it being really handy for testing long forms even across multiple pages etc.
There's a
great video demo which quickly shows you what it does and it's only a light download and had it working in seconds.
Selenium also comes as HTML page so you can test without the plug-in check out this
example.
Notably it's the guys from
Thoughtworks that wrote this who also wrote CruiseControl.NET which I've been using for a few months now.