Backbone supplies structure to JavaScript-heavy 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 application over a RESTful JSON interface.

backbone-couchdb is a plugin for Backbone.js that takes care of synching (Backbone-) Models and Collections automatically with a CouchDB instance.

eyeballs.js is a slim javascript library designed to sit on top of a javascript framework, such as jQuery or Prototype

batman.js is a full-stack microframework extracted from real use and designed to maximize developer and designer happiness. It favors convention over configuration, template-less views, and high performance by simply not doing very much. It all adds up to blazingly fast web apps with a great development process; it's batman.js.

Agility.js is an MVC library for Javascript that lets you write maintainable and reusable browser code without the verbose or infrastructural overhead found in other MVC libraries.