Semantic UI - A UI Component library implemented using a set of specifications designed around natural language
Bootstrap - Simple and flexible HTML, CSS, and JS for popular UI components and interactions
jQuery UI - Curated set of user interface interactions, effects, widgets, and themes built on top of the jQuery JavaScript Library
Metro UI CSS - Metro UI CSS a set of styles to create a site with an interface similar to Windows 8.
UIKit - A lightweight and modular front-end framework for developing fast and powerful web interfaces
Tailwind CSS - A utility-first CSS framework for rapidly building custom user interfaces.