A working colleague of mine showed me a tool, which I think is really cool for automating tasks during web application security test, even if it’s not made especially for that. The tool I’m talking about is called Selenium Remote Control and licensed under the Apache 2.0 License. Let’s have a look at the description of Selenium:
Selenium Remote Control (RC) is a test tool that allows you to write automated web application UI tests in any programming language against any HTTP website using any mainstream JavaScript-enabled browser.
This is exactly what Selenium is/does 
I can especially recommend the Selenium IDE, which’s a Firefox extension that can record all actions of your browser, replay them and also export the whole thing into HTML, Java, C#, Perl, PHP, Python and Ruby.
This is, how the Selenium IDE looks like:
I don’t want to say more on this, just give it a try and have fun
Hey Sven, nice to see Selenium catching on a bit. You might want to check out dre’s and my ShmooCon 08 presentation, Path X; Explosive Security Testing with XPath, where we talk about using Selenium and XPath to find vulnerabilities in web applications.