moo.fx is a superlightweight, ultratiny, megasmall javascript effects library, written with prototype.js.
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Building upon the wonderful Prototype JavaScript library, script.aculo.us provides you with some great additional ingredients to mix in. script.aculo.us
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The original Prototype. A JavaScript framework that aims to ease development of dynamic web applications.
And now there is Dojo:
Dojo is the Open Source, portable JavaScript toolkit that lets you build interactive modules quickly, animate transitions, and construct Ajax requests with ease. We think Dojo is the most powerful and easiest to use Ajax and DHTML toolkit on the planet.