I found YUI very helpful for my applications. It’s not the most complete framework that exists (look at BackBase for example) but it is very promising. You can make new Components easily ( with YAHOO.util.Dom, YAHOO.util.Event ), ajaxified behaviour (with YAHOO.util.Connect.asyncRequest ) etc…
I recommand two web sites :
- YUI Home Page (examples, good documentation and downloads)
- Jack Slocum’s Blog (Using the YUI library - Beyond the Examples)
For WordPress users, you can even use YUI to for the comments system.