Bootstrap - Simple and flexible HTML, CSS, and JS for popular UI components and interactions
Material UI - A CSS Framework and a Set of React Components that Implement Google's Material Design
Elastic UI - React-based design library made for use with Elastic products
Semantic UI - A UI Component library implemented using a set of specifications designed around natural language
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.
Tailwind CSS - A utility-first CSS framework for rapidly building custom user interfaces.