Bootstrap - Simple and flexible HTML, CSS, and JS for popular UI components and interactions
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.
Materialize CSS - A modern responsive front-end framework based on Material Design
UIKit - A lightweight and modular front-end framework for developing fast and powerful web interfaces
Tailwind CSS - A utility-first CSS framework for rapidly building custom user interfaces.
Skeleton CSS - A Beautiful Boilerplate for Responsive, Mobile-Friendly Development