Kickstart CSS - The CSS Library that Loves to Be Extended
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.
Bootstrap - Simple and flexible HTML, CSS, and JS for popular UI components and interactions
Primer - CSS toolkit and guidelines that power GitHub
Bulma - Bulma is an open source CSS framework based on Flexbox and built with Sass. It's 100% responsive, fully modular, and available for free.