Yahoo! recommends YUI 3. Learn about YUI 3 Get on YUILibrary.com.
The Get Utility provides a convenient API for appending script nodes and CSS files to an existing DOM. This can be useful when loading functionality on-demand; it's also useful when retrieving JSON data feeds across trusted but non-identical domains.
All YUI 2.x users should review the YUI 2.8.2 security bulletin, which discusses a vulnerability present in YUI 2.4.0-2.8.1.

Copyright © 2013 Yahoo! Inc. All rights reserved.