Recently at Cambridge Software, we have been investigating different JavaScript based microframeworks for building rich applications that bring a desktop experience to the Web. Backbone.js is an excellent library, very lightweight, giving you a great deal of flexibility while covering all the foundation-type work. Backbone.js is built around the MVC pattern, it gives you base classes for models, controllers and views.
Read: Backbone.js + PHP (with Silex microframework) + MySQL