HTML5 Cross Browser Polyfills
Great collection of all the shims, fallbacks, and polyfills in order to implant html5 functionality in browsers that don’t natively support them.
Yepnope is an asynchronous conditional resource loader that’s super-fast, and allows you to load only the scripts that your users need.
Grid Style Sheets
GSS reimagines CSS layout & replaces the browser’s layout engine with one that harnesses the Cassowary Constraint Solver.
Myth is a preprocessor that lets you write pure CSS without having to worry about slow browser support, or even slow spec approval. It’s like a CSS polyfill.
Polyfill.js is a library designed to make writing CSS polyfills much, much easier. It’s an abstraction library that takes care of the boilerplate, so you can focus on what your polyfill actually does.
Hand.js is a polyfill that will allow you to write your code about touch only once even if the browser does not support pointer events.
Smashingmag wrote very nice article about perfect polyfill solution for responsive images that you can find here.