jQuery

jQuery is a fast, small, and feature-rich JavaScript library. It makes things like HTML document traversal and manipulation, event handling, animation, and Ajax much simpler with an easy-to-use API that works across a multitude of browsers. With a combination of versatility and extensibility, jQuery has changed the way that millions of people write JavaScript.

Due to some useful jQuery plugins are outdated and don't work properly with jQuery v.2.xx Included is also line of code to load jQuery v1.xx on demand, Otherwise jQuery v2.xx is loaded.

Homepage: http://www.jquery.com