Bootflat - BOOTFLAT is an open source Flat UI KIT based on Bootstrap CSS framework. And, for the designers, we offer a free PSD Download. It provides a faster, easier and less repetitive way for web developers or designers to create elegant web apps.
Primer - CSS toolkit and guidelines that power GitHub
Metro UI CSS - Metro UI CSS a set of styles to create a site with an interface similar to Windows 8.
UIKit - A lightweight and modular front-end framework for developing fast and powerful web interfaces
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.
Tailwind CSS - A utility-first CSS framework for rapidly building custom user interfaces.