d3wp

This plugin generates nice jQuery powered sliding contact form in the left border of your WP blog. 
Contains client side validation and ajax powered mailing, so no page refresh is required. 
Admin section allows you to configure all form labels including error messages and form status messages.
<h3>Feedback</h3>
http://www.webdev3000.com/

WD3K Ajax Sliding Contact Form

An Ajax powered sliding contact form, based on Contactable (jQuery Plugin) By Philip Beel.

aposai

Display your archives in collapsible mode using ajax and jQuery.
Each subnode is loaded via ajax as per user request, so not whole archive is loaded each time your archives page is displayed. If you have performance problems or server issues with other archive plugins, try this one and enjoy.
It also displays the post count for each year and for each month and the comment count for each post. And uses your current wordpress date_format to format the dates.

Ajax Archives

Display your archives in collapsible mode using ajax and jQuery.

WD3K Ajax Sliding Contact Form vs Ajax Archives