Whether you want to experiment with responsive design elements, or you’re ready to create some responsive HTML prototypes, Wirefy will let you get started quickly.
From the site:
Wirefy is a collection of CSS & JS files to help you rapidly experiment with responsive wireframes. Whether you keep them to yourself or use them to help educate your clients, Wirefy is flexible and here to help. Wirefy has been built from the ground up. Following the philosophy of mobile first, Wirefy will respond to the proper viewport.
Very nice work and a huge help for those who want to get some first hand knowledge of how a responsive layout will work in the wild.