jQuery UI

jQuery UI is a large collection of interface interactions, effects, widgets, and themes that are built using jQuery. jQuery UI is mostly maintained by volunteers who work on its API design, visual design, implementation, ticket triage, bug fixing, developer relations, infrastructure, and more.

You can get jQuery UI at jqueryui.com. It is built around a modular architecture that allows developers to download only what they need for their application (or download all its components). It comes as a zip file to be downloaded. Once extracted on your computer, you will find a JavaScript library and CSS files included.

The challenging part is determining what files you will need and where to place them on your web server. The time spent copying files and getting your jQuery UI code to work, can save you a lot of time in the long-run by avoiding having to develop the newly added feature from scratch.