ExpressJS - Sinatra inspired web development framework for node.js -- insanely fast, flexible, and simple
SpiroKit - Prototype your next app in one sitting
Chart.js - Easy, object oriented client side graphs for designers and developers.
Marvel - Turn sketches, mockups and designs into web, iPhone, iOS, Android and Apple Watch app prototypes.
Famous Engine - The Famous Engine is a free and open source JavaScript rendering engine.
Flinto - The prototyping tool designers have been waiting for.