Quite by accident, after upgrading to Chrome 23 today, I found that there are now pixel rulers shown by default when inspecting an element.
Apparently this comes from a WebKit update which introduces rulers to Web Inspector. When you open the Elements panel and hover over elements or when you use the magnifying glass they show up.
According to Masataka Yakura, the guy who brought this into Chrome, he found out that people didn’t quite like this. In Chrome 25 you can disable the rulers via the preferences panel. But personally I like them!