Javascript is a powerful tool for creating dynamic web content.  Web browsers have become faster and more powerful over time and the primary access to a web browser is through javascript.

There are a lot of javascript libraries that are opensource and can simplify some of the more difficult tasks performed within the web browser.

Some of the most common are:

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  jQuery - jQuery is one of the most used & most powerful javascript libraries and creates a foundation for handling virtually any browser event or task
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  jQuery UI - jQuery UI is the user interface that goes along with the jQuery library providing many controls and styles
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  YUI Library - YUI is a javascript library that simplifies most of the the more complex browser tasks that can be accomplished using javascript