Google has open sourced their internal JavaScript library under the name Closure.
You can download the library using any svn client by pointing to http://closure-library.googlecode.com/svn/trunk/ and getting everything locally into a closure-library-read-only folder.
All your custom html/js can then reside next to this folder.
To use the framework you have to load the base.js into your document.
<script src="closure-library-read-only/closure/goog/base.js">
Additionally you can import specific class by registering the namespace in your javascript code file like this:(yes its very object oriented!)
goog.require('goog.dom');
goog.require('goog.ui.Zippy');
My folder structure looks like this: (with the hello and notepad test projects)
To further browse the samples open your \closure-library-read-only\closure\goog\demos\index.html file.
It demos the power of the framework with some really cool samples.
So far I love what am seeing. Hope to build something cool real soon!