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.
I've just started to dive into this library - and I gotta say, I'm loving it. At just 5.3kb it really packs a punch, and gives a solid Model-View framework to your app. It comes equipped with an impressive knowledge base, and from the limited time I've spent with it, the learning curve is small.
Over the next few weeks I'll be sharing a lot more on how to organize your front-end delivery with backbone, and create stunningly fast, rock-solid apps using this new(ish) library.