Bootstrap - Simple and flexible HTML, CSS, and JS for popular UI components and interactions
Metro UI CSS - Metro UI CSS a set of styles to create a site with an interface similar to Windows 8.
Element UI - A Desktop UI Library. Element, a Vue 2.0 based component library for developers, designers and product managers.
Tailwind CSS - A utility-first CSS framework for rapidly building custom user interfaces.
Responsive Grid System - Spectacularly Easy Responsive Design
Materialize CSS - A modern responsive front-end framework based on Material Design