ExtJs
Jack Slocum, author of YAHOO.Ext, has recently launched http://extjs.com/. Initially, I thought the site was a remake of his personal site http://jackslocum.com/ with a greater focus on his YUI extension library. However, upon closer inspection, I realized the new website is dedicated also to framework interoperability. And since that is one of the main goals of this blog, I felt it worth mentioning. Anyway, Ext already works with jQuery, Prototype, and Scriptaculous.
Note: Ext is now also works standalone, without any other library. Although, I am impressed with the work this team has put into Ext, it is still to heavy for most websites. Using this Framework, it is very easy to create JavaScript libraries of 300k or more.