SpiroKit - Prototype your next app in one sitting
ExpressJS - Sinatra inspired web development framework for node.js -- insanely fast, flexible, and simple
Marvel - Turn sketches, mockups and designs into web, iPhone, iOS, Android and Apple Watch app prototypes.
Chart.js - Easy, object oriented client side graphs for designers and developers.
Flinto - The prototyping tool designers have been waiting for.
Backbone.js - Give your JS App some Backbone with Models, Views, Collections, and Events