I've been developing a responsive theme for a new site, and I've found Responsive Design Bookmarklet extremely useful. Just drag the button from the site to the bookmark bar in your browser and when you activate it, you'll have the ability to shrink the viewport to a number of pre-set widths. It's a great way to quickly test how your site design will look when constrained by the smaller screen sizes of mobile and tablet devices (in both vertical or horizontal orientations).