<p>What's the best, crossplatform way to perform blackbox tests on AJAX web applications?</p>

<p>Ideally, the solution should have the following attributes:</p>

<ul>
<li>Able to integrate into a continuous integration build loop</li>
<li>Cross platform so I you can run it on Windows laptops and Linux continuous integration servers</li>
<li>Easy way to script the interactions</li>
<li>Free-as-in-freedom so you can adapt it into your tool chain if necessary</li>
</ul>

<p>I've looked into HttpUnit but I'm not conviced it can handle AJAX-heavy websites.</p>

Selenium might be what you're looking for:
http://selenium.openqa.org/
It allows you to script actions and evaluate the results. It's open-source (Apache 2.0), cross platform, and has nice tools.