While developing advanced and complex Ajax applications it's interesting to have in hands a solution to create automated tests for UI, performance, load ... etc. Here is two professional solutions, and let me know if you know others.

Squish, Cross Platform Ajax testing solution

Froglogic have just released Squish 3.3 beta, a cross platform automated GUI and Web testing solution. Squish is available in different editions for the web, Qt, Java, Mac, 4Js, Tk and XView. The advantage of Squish web edition is its ability to test html and ajax-based web applications in different web browsers and different platforms : IE, Mozilla, Firefox, Safari, Konqueror, ... Squish use a scripting language to automate testing suites, and to create complex testing scenarios, but I'm not sure if it's able to make stress test for Ajax applications. Squish also lacks in graphic reporting. pricing start from 1,600 EUR (about $2300 US).

WAPT 5.0, load, stress and performance testing for the web 2.0

I have been using WAPT since its version 4.0, a web application load, stress, and performance testing solution. In that time their CEO told me that Ajax will be supported in future releases, most probably, in the next version of WAPT 5.0. Even if it's not highlited on their website, Ajax is already on their features list. We have a detailed review of WAPT that you might check for more details on this solution. WAPT is available for windows only and pricing start from $350 US.