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.

TCBarrett

This plugin requires is just the front end javascript library, the following two things to be done in support: 
1. Add the class ‘simpleajaxform’ to your form. 
2. Make sure you have the correct wp_ajax handler hook defined.
It was primarily built as an example for a blog post.

Simple AJAX Form

A simple jQuery javascript library to ajax enable forms.

WD3K Ajax Sliding Contact Form vs Simple AJAX Form