I've been looking for something like this for a while. Looks very cool!
Hey everyone! I've been working on this for a long time! It's a browser specifically for designers and developers making responsive websites (so, everyone ;) Check the video for a rundown, but Polypane lets you see your site in many different screen sizes at once, gives you a quick export of all screen sizes so you can get signoff quickly, and contains the developer tools you're used to from Chrome!