My current project at work is implementing an automated GUI test harness using Selenium. If you're doing any sort of serious Web development, you owe it to yourself to check this thing out.
It has all the usual virtues and vices of open source. The documentation is a tad crappy, and there are some rough edges. OTOH, it's free, not terribly hard to integrate and use, and impressively powerful. It runs with both Firefox and IE, and the Remote Control system has integration modules for many major development platforms (Java, .NET, Perl, PHP, Python, Ruby, etc).
In a bit over a week of work, I should be able to set up enough harness and examples to hand the thing off to QE and have them continue to automate the GUI-level testing from there. Combine that with the API and Unit test suites we already have, and I'm starting to think we may actually be able to pull off a reasonably agile development style. This tool looks to be exactly what we needed at this point; take a look at it...