Buried in there is a tool for a quick view of any site across laptop, tablet and smartphone form factors:
I've never seen this tool before and don't know if it's new with this site, but it is amazing and I'll be using it a lot from now on!
Edit: Thanks for the references to the browser dev tools, which clearly are more appropriate for development - I intended rather more emphasis on the quick view part being the amazing thing, as in lining the three up side-by-side with a nice skeuomorphic ui, and being able to share a link that just works for demos to people not familiar with their browser's dev tools.