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.
Foundation - The most advanced responsive front-end framework in the world
Milligram - A minimalist CSS framework
Bootstrap - Simple and flexible HTML, CSS, and JS for popular UI components and interactions
React95 - Refreshed windows95 UI components for modern web apps
Materialize CSS - A modern responsive front-end framework based on Material Design