KnockoutJS: KnockoutJS is one of the best things that can happen to a web developer. Starting to learn KnockoutJS gives you the equivalent feeling which you would feel at the very moment when you find a cup of hot chocolate in a freezing cold day. It enables your to have two way data-binding between your DOM elements and your view model which means when your data model's state changes, your UI updates automatically and vice versa. Check out its well-arranged documents and live tutorials.
Sammy.js: Last library I am considering for my every web application is Sammy.js. It gives us a nice way to handle page navigations without refreshing the page at all by providing a nice routing mechanism. Especially, if you are writing a Single Page Applications (SPA), this is the library that you want inside your suite. Don’t skip to check out its intro sample.