Bootstrap - Simple and flexible HTML, CSS, and JS for popular UI components and interactions
Elastic UI - React-based design library made for use with Elastic products
Tailwind CSS - A utility-first CSS framework for rapidly building custom user interfaces.
Semantic UI - A UI Component library implemented using a set of specifications designed around natural language
Materialize CSS - A modern responsive front-end framework based on Material Design
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.