I've started play around a little bit with Watir (pronounced "water" and stands for Web App Testing in Ruby), lately after some colleagues at Immediacy gave me a small demo on it. It is essentially an open source library for automating the web browser and can be used for testing web applications.
I am not going to go into detail about what it is cos' there is loads of information out there already but I was quite impressed by what Watir could do. My main concern was that you needed to hand craft your automation scripts. Now I am sure a bit of effort up front would pay dividends in the long run but it just felt like a little bit too much work to get the scripts up and going, that is until I came across this handy little tool. Once again Scott Hanselman seems to come to the rescue by developing the first version called WatirMaker, opening up the source code to the community and then a couple of others take it on and enhance it.
I've only used it very briefly but it is pretty cool and worth checking out.