We use Sass to compile our CSS. Typically we use the rem unit for everything.
Gulp can be used to run a post-compile fix to create fallbacks for all of our rem declarations that are neccessary for IE8.
Respond is a library that allows IE6-8 to read media queries which is essential since we are using a grid system. This should only be loaded conditionally if you are trying to support those browsers.
Modernizr is used for feature detection. It also ships with html5shiv.js which enables HTML5 elements in IE.
We use the basic grid logic from the Foundation library in order to create our grid system.