RWD & Mobile Tools
The popular responsive images polyfill allows you to use HTML's new <picture> element and its associated srcset and sizes attributes in your projects today, with strong browser support.
There are loads of responsive layout testing tools, I'm only including one, because it's all you need. ish does exactly what it should do, without an unnecessary focus on device sizes.
A simple online tool that lets you grab responsive embed codes for all sorts of popular media sources. It supports YouTube, Vimeo, Google Maps, Instagram, Vine and more.
Responsive tables are one of the most difficult layout considerations. Tablesaw is a set of jQuery plugins to help in this area, by means of features like stack, toggle, minimap and more.
A bookmarklet that provides a tool panel with which you can overlay an image of a design (with alpha setting) to compare to your work in the browser. You can also test breakpoints.
A Grunt plugin to automate tasks associated with responsive images: image resizing, rasterising of SVGs and PDFs, and lossless optimisation.
40. Email Framework
An HTML email framework to help build responsive email layouts and templates. This one offers well-tested modules, grids, typography, buttons and more.
Next page: free SVG tools and best of the rest...