Knockout (http://knockoutjs.com/) definitely rocked my world. Forget about manipulating the DOM with code (using jQuery or anything): now you have a nice separation between your UI (defined declaratively) and your business logic. Just change some observable values and voila, your UI is updated. I strongly recommend trying the site's tutorial: http://learn.knockoutjs.com

Also if you like Backbone.js, don't miss Knockback.js (http://kmalakoff.github.com/knockback/). Now you have observable (view)models and collections. Definitely cool.