Backbone.js Gives structure to web applications by providing models with key-value binding and custom events, collections with a rich API of enumerable functions, views with declarative event handling, and connects it all to your existing API over a RESTful JSON interface.
TogetherJS A free, open source JavaScript library by Mozilla that adds collaboration features and tools to your website. By adding TogetherJS to your site, your users can help each other out on a website in real time.
Hoodie Hoodie is an Offline First and noBackend architecture for frontend-only web apps. Enables you to build complete web apps in days, without having to worry about backends, databases or servers, all with an open-source library that's as simple to use as jQuery.
Two.js A two-dimensional drawing api geared towards modern web browsers. It is renderer agnostic enabling the same api to draw in multiple contexts: svg, canvas, and webgl.