Backbone gives you a minimalist, un-opinionated structure to web applications by providing models, collections and views and connects it all to your existing REST API.
Ember.js takes a different approach, offering a highly structured, convention over-configuration approach to web development that borrows heavily from the philosophy behind projects like Ruby on Rails.
Angular offers a unique approach to application structure, attempting tomimic the future of current web standards, but often at the expense of a steep learning curve.
jQuery Mobile is a unified, HTML5-based user interface system for all popular mobile device platforms.
Bootstrap and Foundation are powerful responsive, mobile-first front-end frameworks for faster and easier web development.
Polymer is a new type of library for the web, built on top of Web Components, and designed to leverage the evolving web platform on modern browsers.
HTML5 Boilerplate is a collection of best practices and experience from hundreds of developers to provide a solid starter template for modern web apps.
HTML5 Project Builder is a fork available on Github allowing you to create a backend for any client side website.