Left to our own devices. — Ethan Marcotte
Your website’s only as strong as the weakest device you’ve tested it on.
Bricss - Simple responsive design test page
A handy little document to load pages into differently-sized iframes—useful for testing responsive designs.
Related links
Emmet Re:view — fast and easy way to test responsive design in multiple viewports
It’s no substitute for testing with real devices, but the “device wall” view in this Chrome plug-in is a nifty way of getting an overview of a site’s responsiveness at a glance.
embedresponsively.com
Here’s the CSS and markup you need to make third-party iframes responsive. Handy!
Responsive Inspector
A handy plugin for Chrome that always you to inspect media query breakpoints and take screenshots at any of them.
Responsive Design Testing
Another browser-based tool for viewing the same site at different sizes, but this one displays them all the same time, side by side.