Bootstrap framework
Bootstrap is open source and most popular front-end framework for developing responsive, mobile first projects on the web. It's hosted, developed, and maintained on GitHub.
Bootstrap have responsive grids and many components to build a responsive website quickly. Boostrap has much reusable components built to provide iconography, dropdowns, input groups, tootip, navigation, alerts, and much more. By default Boostrap has awesome custom jQuery plugins and it's very easy include all, or one by one.