@lensco made an excellent post a couple of days ago that showed off a simple, but elegant responsive design test page that loads the same content into multiple iframes for easy viewing. See:

Great idea. Here’s a bookmarklet version of the same code that lets you use it on any webpage:

EDIT: I got stuck waiting for a flight today, so I update the script to add a little JS goodness. Now it lets you specify which viewport sizes you’re interested in (or supply your own dimensions), and generate a custom bookmarklet. Neat.