Backbone.js is catching on and for good reason. It’s not heavy weight like SproutCore or Cappucino, gets you going quickly, and gets out of your way. Some folks complain that is’t not full-featured enough. But that’s because it’s designed to be light weight and über extensible. It doesn’t do what you want? Extend it! Using it’s own built in .extend or with the now 100% completely indispensible underscore.js, it’s only required dependecy.