MJML App for OS X makes it easy to create responsive emails on your desktop. You can import and manage templates, edit emails with live rendering, and more.
LettError gives you scalable, interpolating vector shapes that can make themselves fit in a range of rectangles. It depends on Snap.svg and jQuery.
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UnMQ removes media queries from your CSS while preserving rules that match a hard-coded viewport. It's useful for outputting CSS for older desktop browsers, like IE8.
Avalanche is a responsive, Sass-based CSS system with flexible, configurable naming conventions. It uses real-world breakpoint naming and integrated media query mixins.
Cerberus is a set of simple, solid patterns for responsive HTML emails. They even work in Gmail and Outlook.
ResponsifyJS makes truly responsive images while taking into account the content of photos. You can define the focus area or let it define it automatically.
Wraith is a responsive screenshot comparison tool. It's well-documented and easy to install.
Skel is a lightweight, responsive framework for building sites and apps. It includes modules for layout, viewport management simplification, and more.