Yahoo! has released a free library of AJAX-enabled UI components, including a set of core utilities for animation, drag-and-drop, events, DOM, as well as calendar, slider, and treeview controls.  The components are both free and open-source under a BSD license.

Also noteworthy is the new Yahoo! Design Pattern Library.  Here they attempt to demonstrate UI best practices for the Web including breadcrumbs, pagination, and other common UI elements.  This is their first installment of what they promise to be a monthly installment of the latest Web patterns and practices.