Cuzillion is a tool for quickly constructing web pages to see how components interact. Browsers have unexpected behavior in everyday situations (for example, inline scripts block all rendering in the page and nothing can download in parallel with an external script). Sometimes the behavior differs across browsers (Internet Explorer 8 downloads six items in parallel whereas Firefox 2.0 only downloads two in parallel). Cuzillion lets you observe these behaviors and share sample pages with others

Dion did a quick interview with him which contains a good explanation of how it works.

As per the screencast Steve walks us through an example of how Cuzillion could be used:

A great example of how Cuzillion is useful is looking at the impact inline scripts have when they follow a stylesheet in Internet Explorer. Typically, a stylesheet followed by any other resource results in both resources being downloaded in parallel in Internet Explorer. (In Firefox stylesheets block parallel downloads, so this performance optimization is only applicable to IE.) Here’s a Cuzillion page that shows this: stylesheet and image downloading in parallel. Both the stylesheet and image are configured to take 2 seconds to download. Since they download in parallel the page takes about 2 seconds to load as shown by the “page load time”.

But look what happens if we put an innocent inline script between the stylesheet and image: stylesheet, inline script, and image. Now, in Internet Explorer the stylesheet and image are downloaded sequentially, which means the page load time goes from 2 seconds to 4 seconds. If the inline script is simply moved above the stylesheet the two resources are downloaded in parallel again, and the page load goes back down to 2 seconds: inline script, stylesheet, and image.

Read more about it on Steve's blog.

I think it's great tool for developers to optimize their application load times and also could be used as an educational tool for new developers as well.