Node.js

Node.js is the Server Side Javascript that is famous for making networking applications like Chat Systems and Web Apps.

Node.js has a short Learning curve and is great if you already know Javascipt.

Web Frameworks like Express, Sails, etc. are available for free and can be deployed on Heroku Cloud Computing Platform.

Javascript is the most easiest language to work with and whole lot of plugins and UI libraries available for free as well as paid.

JQuery, JQuery UI, JQuery Widgets, Backbone, Angular, Knockout, Twitter Bootstrap etc are all top Javascript Libraries.