with which eg. Gmail, Google Maps, Google Docs are build.
Go to http://code.google.com/closure/.
Download the code with a Subversion client like RapidSVN.
Start RapidSVN and follow the next steps.
1. Go to menu 'Bookmarks',
choose 'Add Existing Repository...'
in the 'Repository URL'-window enter as 'URL' http://closure-library.googlecode.com/svn/trunk.
These steps result in the following picture
2. Select the newly created bookmark,
choose menu 'Repository',
and click 'Checkout'.
Specify a 'Destination Directory' in the 'Checkout'-window,
In the end you'll find all the files in the given folder.
Now start with the Getting Started with the Closure Library.
What are u waiting for?