PowerCSS - JS-powered run-time CSS
Bootstrap - Simple and flexible HTML, CSS, and JS for popular UI components and interactions
Primer - CSS toolkit and guidelines that power GitHub
UIKit - A lightweight and modular front-end framework for developing fast and powerful web interfaces
Metro UI CSS - Metro UI CSS a set of styles to create a site with an interface similar to Windows 8.
Tailwind CSS - A utility-first CSS framework for rapidly building custom user interfaces.