Tammy Hart

This plugin adds a page to the admin under Users that includes a form for users to send email invitations to their friends to register at your site. You can also add the form to the front end of your site with the <code>invite_friends()</code> template tag or the <code>[invite_friends]</code> shortcode.
IMPORTANT: This plugin will not allow invited friends to register unless you have registration turned on.
<h4>Features</h4>
<ul>
<li>Invite multiple friends at one time</li>
<li>Customize the email subject and message that is sent</li>
<li>Customize the URL to redirect a user to after they register</li>
<li>Store successful registered users in the inviter’s user meta data</li>
<li>Store the inviter in the new user’s meta data</li>
</ul>

Invite Friends to Register

Use this plugin to allow users to invite their friends to register on your site.

Import contacts from the Address Book from Yahoo!, GMail, AOL, Hotmail and other providers. 
The plugin also provides all the nice features of OpenInviter:
<ul>
<li>auto-updates</li>
<li>both local and remote debugging</li>
</ul>
Supported services:
<ul>
<li>AOL</li>
<li>GMail</li>
<li>Windows Live (Hotmail)</li>
<li>Yahoo!</li>
<li>Rediff</li>
<li>Lycos</li>
<li>Mail.com</li>
<li>Mail.ru</li>
<li>Rambler.ru</li>
<li>Twitter</li>
<li>MySpace</li>
<li>Facebook</li>
<li>Hi5</li>
<li>Orkut</li>
<li>GMX.net</li>
<li></li>
</ul>
Services coming soon:
<ul>
<li>GMX.com</li>
<li>Email.it</li>
<li>Fastmail.fm</li>
<li>Canada.com</li>
<li>YouTube</li>
<li>(and many others…) 🙂</li>
</ul>

OpenInviter for WordPress

Allow your visitors to invite their contacts from Yahoo!, GMail, AOL, Hotmail and other providers to your blog.

Invite Friends to Register vs OpenInviter for WordPress