Introduction
It's simple. If you are in the URL bar on a long URL and want to cut to the root, just hit Control + Left Arrow.
This idea came from a blog entry.
Example
If you take a look at this example, although the first URL was precise, I just wanted to get to ajaxian.com.
The problem is that there are often two use cases, either show me based on frequency (e.g. you go to ajaxian.com all the time), or show me based on the last items that you visited in this domain (which is nice for 'what was I just at').
Download
You can grab the extension here