Spine is even lighter than Backbone. I like Spine because its functionality is not really tied to the DOM or jQuery -- I make native mobile apps with Spine on Titanium, for instance. Spine is built out of components (i.e. Model and Router), which can be used separately or in concert. It's a massively underappreciated framework that has had staying power.

Not to mention, stepping through (or trying to log) the endless anonymous functions created by underscore can be a hassle and straight crashes chrome/node inspector in some situations.