bbodine1

Refer notes is a WordPress plugin that is used to display a custom welcome message to site visitors who are referred to your site from other specific site domains such as Digg, Reddit and Stumbleupon.
When a visitor comes to your site from one of the many social networks, a message will be displayed across the top or your site welcoming them from the site that sent them to you. For example, if a user stumbled to your site from stumbleupon, a message saying,”Welcome Stumbleupon user! If you like this article please consider giving us a thumbs up. Thanks!” Each message can be customized to say anything you want.
Options are available for styling your message and the message container. When activated the container sits accross the top of your page and is fixed so it stays present even if the visitor scrolls down the page. The container has a close button that will allow the visitor to slide it up and out of view.
Currently supports more than 20 social websites and 3 search engines. There are 5 custom referrals that can be added so if one of the sites you would like to support is not here you can quickly add it through the options page.
Alternately, you can use the New User option to display a message to any site visitor being referred to your site from any other site except your own because you don’t want it popping up every time change pages around your site.
Some sites like Digg and Reddit support custom buttons to display the current Diggs or Points for a specific page or article. Options are available to use a large or small button or disable this feature all together. Most current sites include a custom favicon type icon specific to the referral site.

Refer Notes

Displays a custom message at the top of your site to welcome users who are referred from other specific sites such as Digg, Reddit and Stumbleupon.

wplift

Super Refer A Friend is plugin which adds a new widget to your blog which allows readers to email the page they are on to a friend via a popup box. 
They can enter more than one email address and customise the message that is sent. 
The plugin includes a captcha and you can set a mailing limit to prevent spam
Support at http://wplift.com/refer-a-friend-wordpress-plugin

Super Refer A Friend

Creates a widget to send the current page to a friend via email with a message.

Refer Notes vs Super Refer A Friend