Tailor mentioned “two of my favorites example are here: http://susy.oddbird.net/ and here: http://designmodo.github.io/Flat-UI/. These are libraries I use all the time in Sass that allow me to shorten repeated CSS for grid elements and UI elements to things like this: $formcolor. Wherever I want to have rounded corners or a grid element that spans three columns responsively I can drop in a @include and include large pieces of CSS with a few keystrokes.
Sass has some good examples here that I can elaborate on tomorrow: http://sass-lang.com/.
Another awesome feature is that preprocessors enabling libraries of code that you can reference easily.