[ByteMuse]

I have been using Backbone for a while and love it. It is lightweight, is as flexible as you want and helps to organize code.

Here is a good HN post on Backbone and Sproutcore, with a great comment by the creator of Backbone: http://news.ycombinator.com/item?id=2119704

[shadowz]

Second Backbone. I use Underscore + Backbone + jQuery and it works really well.

[theitgirl]

I use Underscore + Backbone + jQuery as well.