Have a need for an application that provides a fluid user experience, like a desktop application, but that needs to live on the web? A single-page application, or SPA, is your solution. SPAs load all required code in the browser at once, then dynamically update without ever reloading or leaving the page.
AngularJS, developed and maintained by Google, is the most well-known open source SPA framework. It is a mature library for the most complex and demanding web applications, where custom routing/URLs, analytics, browser history, and application testing are important.