AjaxAnywhere
From Ajaxian.com:
AjaxAnywhere is designed to turn any set of existing JSP components into AJAX-aware components without complex JavaScript coding. In contrast to other solutions, AjaxAnywhere is not component-oriented. You will not find here yet another AutoComplete component. Simply separate your web page into multiple zones, and use AjaxAnywhere to refresh only those zones that needs to be updated.
This is one of the few Ajax projects I've seen that looks to provide a lot of value w/o a whole lot of work. Notice how much it enhances JSF with this cool demo.