Hudson http://hudson-ci.org/ is a continuous integration server - it runs and monitors ‘jobs’ in a way that is useful to regularly build software and report on any errors.

Selenium http://seleniumhq.org/ is a suite of tools specifically for testing web applications - it tests the full website by automating the running of one or more browsers (so you can test all that pointy clicky ajaxy stuff)

Oh cool :-)

I just noticed that selenium IDE has the option to export tests as PHP unit tests.