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.
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
Tailwind CSS - A utility-first CSS framework for rapidly building custom user interfaces.
UIKit - A lightweight and modular front-end framework for developing fast and powerful web interfaces
Skeleton CSS - A Beautiful Boilerplate for Responsive, Mobile-Friendly Development