WP Ajax Contact Form, also this plugin sends mail using ajax and gather email list, have options page, custom css and form design usability.
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Are you looking for lightweight Ajax Contact Form. This plugin mainly design for Contact Form, also this plugin will help you to send mail using ajax from your wordpress site. You can design your own contact form from the admin. You can add and remove any type of field in the form on fly. There will be a email list waiting for your review. There are several options for the contact that you can update later. You just need to put shortcode [wpajaxcf] in any page/post. There is also custom css area in the admin.