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.

This is great - the IDE is a good way to interactively build up tests by recording a set of actions and tweaking what it records. 

And being able to save them as unit tests makes it easy to integrate this with automated testing.

Interestingly they are created as PHPunit tests - not simpletest tests.