React is mostly the V in MVC. It focuses entirely on that piece of MVC and disregards the architecture of the rest of your application.
It provides a component layer that makes it easier to make UI elements and combine them together
Backbone provides a full MVC framework along with routing. The Models allow for key-value binding and events for handling data changes. Models (and Collections) can connect to RESTful APIs.
Data visualization and charting is a common requirement for web applications In D3 you to manipulate data documents from any source and apply a transformation into the DOM.
In Babylon.js You can create incredibly high-quality games complete with physics, audio, and particle systems among other things.
It is a lightweight 3D library. that allows rendering 3D to an HTML5 canvas.
Phantom JS allows you to run a headless Webkit. the rendering engine behind Safari and previously Chrome.