* JQuery Form Submit – http://jquery.malsup.com/form/
This will let you submit forms via AJAX automatically that can save you time doing AJAX. This also lets you upload files via ajax and its pretty daunting doing file upload via ajax.
*JQuery Text Resizer – http://plugins.jquery.com/project/TextAreaResizer
This allows your text area to be resizeable its pretty easy to set up.
I will update this list regulary once I come across other useful plug-ins.