I’ve never had any good experiences with tools for automatic testing but I must confess that Selenium looks nice. At least if you want to add regression tests to an existing GUI. And it is a web based GUI. So if those two fact apply to you, you should take a look at Selenium I think.
Selenium is good at recording actions and create test code. And it creates test code for unit tests frameworks such as NUnit and a whole bunch of other languages such as Python, Java, Ruby, PHP and more. However there seems to be some stability problems at the moment. According to the presenter (I have not found a link to verify this); Google have more than 51000 selenium tests in their projects. 96% of the tests run with no problem, 2% have problems due to confirmed bugs in Selenium and the last 2% are tests failing where the reason is unclear (Selenium vs the code tested).
This is a common problem I see when I work with open source. Open source applications are typically quite useful and easy to use 99% of the time. But I always tend to end up needing the last percent which is never implemented or have bugs. And the reason for this is usually that no one have bothered to implement the last tricky advanced feature I end up needing. You probably wanna know why I do not sit down and implement that feature and contribute it back to the open source community. Well that is a whole other topic and I will write about that it the near future.