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From SmashMagazine:

Thanks to the Web’s widespread adoption of JavaScript, JavaScript libraries have sprung up to help make design and development easier. Here are a few of the major JavaScript libraries that developers use: jQuery, Prototype, Scriptaculous, mootools, Dojo. These frameworks have thriving communities whose members have developed countless plug-ins that can greatly add to the JavaScript framework.

Personal favorites include:

Facebook Autocompletion

Galleria

Coda sliding tabs

amCharts

See all of the useful javascript techniques