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.

frontierthemes

Plugin that enables content sharing by means of social unlocking. It allows you to lock content you deem interesting and then allowing your public to unlock said content by sharing the URL of your website along with a unique identifier.
<h3>Copyright</h3>
ProLocker WordPress Plugin, Copyright 2020 Frontier Themes. 
ProLocker WordPress Plugin is distributed under the terms of the GNU GPL.
ProLocker bundles the following third-party resources:
React, Copyright (c) 2013-2020 Facebook, Inc., Copyright (c) 2013-2020 React Team 
Licenses: MIT 
Source: https://reactjs.org/

ProLocker

Plugin that enables content sharing by means of social unlocking. It allows you to lock content you deem interesting and then allowing your public to  …

Super Refer A Friend vs ProLocker