[Edit: Check your browser’s devtools first! As @SkylarIttner points out in the comments, tools for responsive design testing have been rolled out since in most browsers since the below solution was posted. They are likely the best/easiest option now.]
You could, correct me if I’m wrong, simply create an iframe with style="desired width & height" and src="https://stackoverflow.com/questions/6400207/your/test.site" as the only child of <body>. Should display the site as if the resolution was the specified width/height and result in scroll bars to examine it.
Not as convenient as using a third party, having to set it up yourself, but has the advantage of being able to test locally with no internet connection.