Prototype takes the complexity out of client-side web programming. Built to solve real-world problems, it adds useful extensions to the browser scripting environment and provides elegant APIs around the clumsy interfaces of Ajax and the Document Object Model.
With the introduction of HTML5 , there is a new HTML element added into the HTML family--Canvas, canvas is very useful as it allows the user to operate as a painting element and enables you to control what you can draw. Processing.js is designed for manipulating canvas elements.
Processing.js makes your data visualizations, digital art, interactive animations, educational graphs, video games, etc. work using web standards and without any plug-ins. You write code using the Processing language, include it in your web page, and Processing.js does the rest. It's not magic, but almost.
Backbone is a framework for managing client side data structure.Based on the classical MVC model Backbone.js gives structure to web applications by providing models with key-value binding and custom events, collections with a rich API of enumerable functions, views with declarative event handling, and connects it all to your existing API over a RESTful JSON interface.
AngualrJS is another MVC framwework powered by Google. It enables the web apps which have many data driven components to be developed quickly.
If you are a developer who lacks of sense of art and want to make you web app online as soon as possible without much intervention in the UI design, you can give a try of Twitter Bootstrap. This framework provides a standard template for creating attractive UIs, you just need to put the necessary classes and tags created by Bootstrap, then you can get waht you want.
Handlebars provides the power necessary to let you build semantic templates effectively with no frustration.