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
Foundation - The most advanced responsive front-end framework in the world
Purecss - A set of small, responsive CSS modules that you can use in every web project.
Semantic UI - A UI Component library implemented using a set of specifications designed around natural language