Reading about jQuery plug-ins and other i was to AjaxLine web site, there i read a fantastic post about 15 most interesting jQuery tutorials.  Some great plug-ins and samples that you will see there:

Create A Tabbed Interface
PHP & Ajax with the Twitter API
Directory Trees With PHP
jQuery Sequential List
Create a jQuery Popup Bubble
AutoCompleter
Create an apple style menu and improve it via jQuery
see more there…

Link: http://www.ajaxline.com/15-most-interesting-jquery-tutorials