I came across a superb testing application called Selenium. You can even use it in the form of a Firefox Add-on. Basically it allows you to record (as you would a macro) functional tests.
For example, you could open the add-on via Tools > Selenium IDE, click record, then navigate to your web application (or anyone elses for that matter), log in, and try submitting a form. Go back to the Selenium IDE, and you will see a list of the commands you just performed, and you can check for certain results. So if you submit a form and don't fill in the required fields, you can check that it returns some text saying "failed". It is capable of much much more, including testing Ajax results etc.
Definitely worth a play!

I tried this out a while ago but it didn't seem to work properly i.e. gave me a couple of incorrect results which scared me off. I'll give it another go.