Responsivator is a web-based app were you can paste in the URL of any page and see how it looks at various screen resolutions.
What’s the point of this? You can just resize your own browser as fast as pulling this up and then side-scrolling. I got kind of excited at first, but the problem is this doesn’t actually show you what it looks like in iPhone. I’m not sure if it just ignores the viewport meta or exactly what’s going on, but the sites I tried look nothing like that in iPhone, not like a few differences but not even close.