Getting HTML from web pages that use AJAX
You may want to look at htmlunit
In The Productive Programmer author Neal Ford suggests that the functional testing tool Selenium can be used for non-testing tasks. Your task of inspecting HTML after client side DOM manipulation has taken place falls into this category. Selenium even allows you to automate interactions with the browser so if you need some buttons clicked to fire some AJAX events, you can script it. Selenium works by using a browser plugin and a java based server. Selenium test code (or non-test code in your case) can be written in a variety of languages including java, C# and other .Net languages, php, perl, python and ruby.
Why choose when you can have both? TestPlan supports both Selenium and HTMLUnit as a backend. Plus it has a really simple language for doing the most common tasks (extensions can be written in Java if need be -- which is rare actually).