UIKit - A lightweight and modular front-end framework for developing fast and powerful web interfaces
PowerCSS - JS-powered run-time CSS
Metro UI CSS - Metro UI CSS a set of styles to create a site with an interface similar to Windows 8.
Primer - CSS toolkit and guidelines that power GitHub
Bootstrap - Simple and flexible HTML, CSS, and JS for popular UI components and interactions
Materialize CSS - A modern responsive front-end framework based on Material Design