Materialize: A CSS framework for looking and breathing Material Design (Google)
Codyhouse.co: Not a framework but a free library for cool html/css/js nuggets (ex. effects, navigation)
CodyHouse - Free HTML, CSS, JS nuggets
Three.js: 3D Javascript library
three.js - Javascript 3D library
Balloon.css: simple tooltip library
Hamburgers: hamburgers galoreee
Spinners: for your loading page needs
CSSCO: if you love VSCO, you'll love this